fix: make error passthrough effective for non-failover upstream errors

backend/internal/handler/gateway_handler.go

@@ -135,6 +135,11 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 	// Track if we've started streaming (for error handling)
 	streamStarted := false
 
+	// 绑定错误透传服务，允许 service 层在非 failover 错误场景复用规则。
+	if h.errorPassthroughService != nil {
+		service.BindErrorPassthroughService(c, h.errorPassthroughService)
+	}
+
 	// 获取订阅信息（可能为nil）- 提前获取用于后续检查
 	subscription, _ := middleware2.GetSubscriptionFromContext(c)