fix: respect OpenAI model mapping in admin available models

backend/internal/handler/admin/account_handler.go

@@ -1718,13 +1718,12 @@ func (h *AccountHandler) GetAvailableModels(c *gin.Context) {
 
 	// Handle OpenAI accounts
 	if account.IsOpenAI() {
-		// For OAuth accounts: return default OpenAI models
-		if account.IsOAuth() {
+		// OpenAI 自动透传会绕过常规模型改写，测试/模型列表也应回落到默认模型集。
+		if account.IsOpenAIPassthroughEnabled() {
 			response.Success(c, openai.DefaultModels)
 			return
 		}
 
-		// For API Key accounts: check model_mapping
 		mapping := account.GetModelMapping()
 		if len(mapping) == 0 {
 			response.Success(c, openai.DefaultModels)

backend/internal/handler/admin/account_handler_available_models_test.go

+package admin
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/Wei-Shaw/sub2api/internal/service"
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/require"
+)
+
+type availableModelsAdminService struct {
+	*stubAdminService
+	account service.Account
+}
+
+func (s *availableModelsAdminService) GetAccount(_ context.Context, id int64) (*service.Account, error) {
+	if s.account.ID == id {
+		acc := s.account
+		return &acc, nil
+	}
+	return s.stubAdminService.GetAccount(context.Background(), id)
+}
+
+func setupAvailableModelsRouter(adminSvc service.AdminService) *gin.Engine {
+	gin.SetMode(gin.TestMode)
+	router := gin.New()
+	handler := NewAccountHandler(adminSvc, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil)
+	router.GET("/api/v1/admin/accounts/:id/models", handler.GetAvailableModels)
+	return router
+}
+
+func TestAccountHandlerGetAvailableModels_OpenAIOAuthUsesExplicitModelMapping(t *testing.T) {
+	svc := &availableModelsAdminService{
+		stubAdminService: newStubAdminService(),
+		account: service.Account{
+			ID:       42,
+			Name:     "openai-oauth",
+			Platform: service.PlatformOpenAI,
+			Type:     service.AccountTypeOAuth,
+			Status:   service.StatusActive,
+			Credentials: map[string]any{
+				"model_mapping": map[string]any{
+					"gpt-5": "gpt-5.1",
+				},
+			},
+		},
+	}
+	router := setupAvailableModelsRouter(svc)
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodGet, "/api/v1/admin/accounts/42/models", nil)
+	router.ServeHTTP(rec, req)
+
+	require.Equal(t, http.StatusOK, rec.Code)
+
+	var resp struct {
+		Data []struct {
+			ID string `json:"id"`
+		} `json:"data"`
+	}
+	require.NoError(t, json.Unmarshal(rec.Body.Bytes(), &resp))
+	require.Len(t, resp.Data, 1)
+	require.Equal(t, "gpt-5", resp.Data[0].ID)
+}
+
+func TestAccountHandlerGetAvailableModels_OpenAIOAuthPassthroughFallsBackToDefaults(t *testing.T) {
+	svc := &availableModelsAdminService{
+		stubAdminService: newStubAdminService(),
+		account: service.Account{
+			ID:       43,
+			Name:     "openai-oauth-passthrough",
+			Platform: service.PlatformOpenAI,
+			Type:     service.AccountTypeOAuth,
+			Status:   service.StatusActive,
+			Credentials: map[string]any{
+				"model_mapping": map[string]any{
+					"gpt-5": "gpt-5.1",
+				},
+			},
+			Extra: map[string]any{
+				"openai_passthrough": true,
+			},
+		},
+	}
+	router := setupAvailableModelsRouter(svc)
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodGet, "/api/v1/admin/accounts/43/models", nil)
+	router.ServeHTTP(rec, req)
+
+	require.Equal(t, http.StatusOK, rec.Code)
+
+	var resp struct {
+		Data []struct {
+			ID string `json:"id"`
+		} `json:"data"`
+	}
+	require.NoError(t, json.Unmarshal(rec.Body.Bytes(), &resp))
+	require.NotEmpty(t, resp.Data)
+	require.NotEqual(t, "gpt-5", resp.Data[0].ID)
+}