refactor: add unified endpoint normalization infrastructure

Introduce endpoint.go with shared constants, NormalizeInboundEndpoint, DeriveUpstreamEndpoint, InboundEndpointMiddleware, and context helpers. This replaces the two separate normalization implementations (OpenAI and Gateway) with a single source of truth. Includes comprehensive test coverage.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode

)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/internal/handler/endpoint.go

+package handler
+
+import (
+	"strings"
+
+	"github.com/Wei-Shaw/sub2api/internal/service"
+	"github.com/gin-gonic/gin"
+)
+
+// ──────────────────────────────────────────────────────────
+// Canonical inbound / upstream endpoint paths.
+// All normalization and derivation reference this single set
+// of constants — add new paths HERE when a new API surface
+// is introduced.
+// ──────────────────────────────────────────────────────────
+
+const (
+	EndpointMessages        = "/v1/messages"
+	EndpointChatCompletions = "/v1/chat/completions"
+	EndpointResponses       = "/v1/responses"
+	EndpointGeminiModels    = "/v1beta/models"
+)
+
+// gin.Context keys used by the middleware and helpers below.
+const (
+	ctxKeyInboundEndpoint = "_gateway_inbound_endpoint"
+)
+
+// ──────────────────────────────────────────────────────────
+// Normalization functions
+// ──────────────────────────────────────────────────────────
+
+// NormalizeInboundEndpoint maps a raw request path (which may carry
+// prefixes like /antigravity, /openai, /sora) to its canonical form.
+//
+//	"/antigravity/v1/messages"   → "/v1/messages"
+//	"/v1/chat/completions"       → "/v1/chat/completions"
+//	"/openai/v1/responses/foo"   → "/v1/responses"
+//	"/v1beta/models/gemini:gen"  → "/v1beta/models"
+func NormalizeInboundEndpoint(path string) string {
+	path = strings.TrimSpace(path)
+	switch {
+	case strings.Contains(path, EndpointChatCompletions):
+		return EndpointChatCompletions
+	case strings.Contains(path, EndpointMessages):
+		return EndpointMessages
+	case strings.Contains(path, EndpointResponses):
+		return EndpointResponses
+	case strings.Contains(path, EndpointGeminiModels):
+		return EndpointGeminiModels
+	default:
+		return path
+	}
+}
+
+// DeriveUpstreamEndpoint determines the upstream endpoint from the
+// account platform and the normalized inbound endpoint.
+//
+// Platform-specific rules:
+//   - OpenAI always forwards to /v1/responses (with optional subpath
+//     such as /v1/responses/compact preserved from the raw URL).
+//   - Anthropic  → /v1/messages
+//   - Gemini     → /v1beta/models
+//   - Sora       → /v1/chat/completions
+//   - Antigravity routes may target either Claude or Gemini, so the
+//     inbound endpoint is used to distinguish.
+func DeriveUpstreamEndpoint(inbound, rawRequestPath, platform string) string {
+	inbound = strings.TrimSpace(inbound)
+
+	switch platform {
+	case service.PlatformOpenAI:
+		// OpenAI forwards everything to the Responses API.
+		// Preserve subresource suffix (e.g. /v1/responses/compact).
+		if suffix := responsesSubpathSuffix(rawRequestPath); suffix != "" {
+			return EndpointResponses + suffix
+		}
+		return EndpointResponses
+
+	case service.PlatformAnthropic:
+		return EndpointMessages
+
+	case service.PlatformGemini:
+		return EndpointGeminiModels
+
+	case service.PlatformSora:
+		return EndpointChatCompletions
+
+	case service.PlatformAntigravity:
+		// Antigravity accounts serve both Claude and Gemini.
+		if inbound == EndpointGeminiModels {
+			return EndpointGeminiModels
+		}
+		return EndpointMessages
+	}
+
+	// Unknown platform — fall back to inbound.
+	return inbound
+}
+
+// responsesSubpathSuffix extracts the part after "/responses" in a raw
+// request path, e.g. "/openai/v1/responses/compact" → "/compact".
+// Returns "" when there is no meaningful suffix.
+func responsesSubpathSuffix(rawPath string) string {
+	trimmed := strings.TrimRight(strings.TrimSpace(rawPath), "/")
+	idx := strings.LastIndex(trimmed, "/responses")
+	if idx < 0 {
+		return ""
+	}
+	suffix := trimmed[idx+len("/responses"):]
+	if suffix == "" || suffix == "/" {
+		return ""
+	}
+	if !strings.HasPrefix(suffix, "/") {
+		return ""
+	}
+	return suffix
+}
+
+// ──────────────────────────────────────────────────────────
+// Middleware
+// ──────────────────────────────────────────────────────────
+
+// InboundEndpointMiddleware normalizes the request path and stores the
+// canonical inbound endpoint in gin.Context so that every handler in
+// the chain can read it via GetInboundEndpoint.
+//
+// Apply this middleware to all gateway route groups.
+func InboundEndpointMiddleware() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		path := c.FullPath()
+		if path == "" && c.Request != nil && c.Request.URL != nil {
+			path = c.Request.URL.Path
+		}
+		c.Set(ctxKeyInboundEndpoint, NormalizeInboundEndpoint(path))
+		c.Next()
+	}
+}
+
+// ──────────────────────────────────────────────────────────
+// Context helpers — used by handlers before building
+// RecordUsageInput / RecordUsageLongContextInput.
+// ──────────────────────────────────────────────────────────
+
+// GetInboundEndpoint returns the canonical inbound endpoint stored by
+// InboundEndpointMiddleware. If the middleware did not run (e.g. in
+// tests), it falls back to normalizing c.FullPath() on the fly.
+func GetInboundEndpoint(c *gin.Context) string {
+	if v, ok := c.Get(ctxKeyInboundEndpoint); ok {
+		if s, ok := v.(string); ok && s != "" {
+			return s
+		}
+	}
+	// Fallback: normalize on the fly.
+	path := ""
+	if c != nil {
+		path = c.FullPath()
+		if path == "" && c.Request != nil && c.Request.URL != nil {
+			path = c.Request.URL.Path
+		}
+	}
+	return NormalizeInboundEndpoint(path)
+}
+
+// GetUpstreamEndpoint derives the upstream endpoint from the context
+// and the account platform. Handlers call this after scheduling an
+// account, passing account.Platform.
+func GetUpstreamEndpoint(c *gin.Context, platform string) string {
+	inbound := GetInboundEndpoint(c)
+	rawPath := ""
+	if c != nil && c.Request != nil && c.Request.URL != nil {
+		rawPath = c.Request.URL.Path
+	}
+	return DeriveUpstreamEndpoint(inbound, rawPath, platform)
+}

backend/internal/handler/endpoint_test.go

+package handler
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/Wei-Shaw/sub2api/internal/service"
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/require"
+)
+
+func init() { gin.SetMode(gin.TestMode) }
+
+// ──────────────────────────────────────────────────────────
+// NormalizeInboundEndpoint
+// ──────────────────────────────────────────────────────────
+
+func TestNormalizeInboundEndpoint(t *testing.T) {
+	tests := []struct {
+		path string
+		want string
+	}{
+		// Direct canonical paths.
+		{"/v1/messages", EndpointMessages},
+		{"/v1/chat/completions", EndpointChatCompletions},
+		{"/v1/responses", EndpointResponses},
+		{"/v1beta/models", EndpointGeminiModels},
+
+		// Prefixed paths (antigravity, openai, sora).
+		{"/antigravity/v1/messages", EndpointMessages},
+		{"/openai/v1/responses", EndpointResponses},
+		{"/openai/v1/responses/compact", EndpointResponses},
+		{"/sora/v1/chat/completions", EndpointChatCompletions},
+		{"/antigravity/v1beta/models/gemini:generateContent", EndpointGeminiModels},
+
+		// Gin route patterns with wildcards.
+		{"/v1beta/models/*modelAction", EndpointGeminiModels},
+		{"/v1/responses/*subpath", EndpointResponses},
+
+		// Unknown path is returned as-is.
+		{"/v1/embeddings", "/v1/embeddings"},
+		{"", ""},
+		{"  /v1/messages  ", EndpointMessages},
+	}
+	for _, tt := range tests {
+		t.Run(tt.path, func(t *testing.T) {
+			require.Equal(t, tt.want, NormalizeInboundEndpoint(tt.path))
+		})
+	}
+}
+
+// ──────────────────────────────────────────────────────────
+// DeriveUpstreamEndpoint
+// ──────────────────────────────────────────────────────────
+
+func TestDeriveUpstreamEndpoint(t *testing.T) {
+	tests := []struct {
+		name     string
+		inbound  string
+		rawPath  string
+		platform string
+		want     string
+	}{
+		// Anthropic.
+		{"anthropic messages", EndpointMessages, "/v1/messages", service.PlatformAnthropic, EndpointMessages},
+
+		// Gemini.
+		{"gemini models", EndpointGeminiModels, "/v1beta/models/gemini:gen", service.PlatformGemini, EndpointGeminiModels},
+
+		// Sora.
+		{"sora completions", EndpointChatCompletions, "/sora/v1/chat/completions", service.PlatformSora, EndpointChatCompletions},
+
+		// OpenAI — always /v1/responses.
+		{"openai responses root", EndpointResponses, "/v1/responses", service.PlatformOpenAI, EndpointResponses},
+		{"openai responses compact", EndpointResponses, "/openai/v1/responses/compact", service.PlatformOpenAI, "/v1/responses/compact"},
+		{"openai responses nested", EndpointResponses, "/openai/v1/responses/compact/detail", service.PlatformOpenAI, "/v1/responses/compact/detail"},
+		{"openai from messages", EndpointMessages, "/v1/messages", service.PlatformOpenAI, EndpointResponses},
+		{"openai from completions", EndpointChatCompletions, "/v1/chat/completions", service.PlatformOpenAI, EndpointResponses},
+
+		// Antigravity — uses inbound to pick Claude vs Gemini upstream.
+		{"antigravity claude", EndpointMessages, "/antigravity/v1/messages", service.PlatformAntigravity, EndpointMessages},
+		{"antigravity gemini", EndpointGeminiModels, "/antigravity/v1beta/models", service.PlatformAntigravity, EndpointGeminiModels},
+
+		// Unknown platform — passthrough.
+		{"unknown platform", "/v1/embeddings", "/v1/embeddings", "unknown", "/v1/embeddings"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			require.Equal(t, tt.want, DeriveUpstreamEndpoint(tt.inbound, tt.rawPath, tt.platform))
+		})
+	}
+}
+
+// ──────────────────────────────────────────────────────────
+// responsesSubpathSuffix
+// ──────────────────────────────────────────────────────────
+
+func TestResponsesSubpathSuffix(t *testing.T) {
+	tests := []struct {
+		raw  string
+		want string
+	}{
+		{"/v1/responses", ""},
+		{"/v1/responses/", ""},
+		{"/v1/responses/compact", "/compact"},
+		{"/openai/v1/responses/compact/detail", "/compact/detail"},
+		{"/v1/messages", ""},
+		{"", ""},
+	}
+	for _, tt := range tests {
+		t.Run(tt.raw, func(t *testing.T) {
+			require.Equal(t, tt.want, responsesSubpathSuffix(tt.raw))
+		})
+	}
+}
+
+// ──────────────────────────────────────────────────────────
+// InboundEndpointMiddleware + context helpers
+// ──────────────────────────────────────────────────────────
+
+func TestInboundEndpointMiddleware(t *testing.T) {
+	router := gin.New()
+	router.Use(InboundEndpointMiddleware())
+
+	var captured string
+	router.POST("/v1/messages", func(c *gin.Context) {
+		captured = GetInboundEndpoint(c)
+		c.Status(http.StatusOK)
+	})
+
+	req := httptest.NewRequest(http.MethodPost, "/v1/messages", nil)
+	rec := httptest.NewRecorder()
+	router.ServeHTTP(rec, req)
+
+	require.Equal(t, EndpointMessages, captured)
+}
+
+func TestGetInboundEndpoint_FallbackWithoutMiddleware(t *testing.T) {
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+	c.Request = httptest.NewRequest(http.MethodPost, "/antigravity/v1/messages", nil)
+
+	// Middleware did not run — fallback to normalizing c.Request.URL.Path.
+	got := GetInboundEndpoint(c)
+	require.Equal(t, EndpointMessages, got)
+}
+
+func TestGetUpstreamEndpoint_FullFlow(t *testing.T) {
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+	c.Request = httptest.NewRequest(http.MethodPost, "/openai/v1/responses/compact", nil)
+
+	// Simulate middleware.
+	c.Set(ctxKeyInboundEndpoint, NormalizeInboundEndpoint(c.Request.URL.Path))
+
+	got := GetUpstreamEndpoint(c, service.PlatformOpenAI)
+	require.Equal(t, "/v1/responses/compact", got)
+}