fix(handler): fallback known gemini models on v1beta 404

backend/internal/handler/gemini_v1beta_handler.go

@@ -121,7 +121,7 @@ func (h *GatewayHandler) GeminiV1BetaGetModel(c *gin.Context) {
 		googleError(c, http.StatusBadGateway, err.Error())
 		return
 	}
-	if shouldFallbackGeminiModels(res) {
+	if shouldFallbackGeminiModel(modelName, res) {
 		c.JSON(http.StatusOK, gemini.FallbackModel(modelName))
 		return
 	}
@@ -674,6 +674,16 @@ func shouldFallbackGeminiModels(res *service.UpstreamHTTPResult) bool {
 	return false
 }
 
+func shouldFallbackGeminiModel(modelName string, res *service.UpstreamHTTPResult) bool {
+	if shouldFallbackGeminiModels(res) {
+		return true
+	}
+	if res == nil || res.StatusCode != http.StatusNotFound {
+		return false
+	}
+	return gemini.HasFallbackModel(modelName)
+}
+
 // extractGeminiCLISessionHash 从 Gemini CLI 请求中提取会话标识。
 // 组合 x-gemini-api-privileged-user-id header 和请求体中的 tmp 目录哈希。
 //

backend/internal/handler/gemini_v1beta_handler_test.go

@@ -3,6 +3,7 @@
 package handler
 
 import (
+	"net/http"
 	"testing"
 
 	"github.com/Wei-Shaw/sub2api/internal/service"
@@ -141,3 +142,28 @@ func TestGeminiV1BetaHandler_GetModelAntigravityFallback(t *testing.T) {
 		})
 	}
 }
+
+func TestShouldFallbackGeminiModel_KnownFallbackOn404(t *testing.T) {
+	t.Parallel()
+
+	res := &service.UpstreamHTTPResult{StatusCode: http.StatusNotFound}
+	require.True(t, shouldFallbackGeminiModel("gemini-3.1-pro-preview-customtools", res))
+}
+
+func TestShouldFallbackGeminiModel_UnknownModelOn404(t *testing.T) {
+	t.Parallel()
+
+	res := &service.UpstreamHTTPResult{StatusCode: http.StatusNotFound}
+	require.False(t, shouldFallbackGeminiModel("gemini-future-model", res))
+}
+
+func TestShouldFallbackGeminiModel_DelegatesScopeFallback(t *testing.T) {
+	t.Parallel()
+
+	res := &service.UpstreamHTTPResult{
+		StatusCode: http.StatusForbidden,
+		Headers:    http.Header{"Www-Authenticate": []string{"Bearer error=\"insufficient_scope\""}},
+		Body:       []byte("insufficient authentication scopes"),
+	}
+	require.True(t, shouldFallbackGeminiModel("gemini-future-model", res))
+}