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。
 //