feat: route AccountTypeUpstream to ForwardUpstream in Forward() entry

Without this routing guard, ForwardUpstream is never called because
Forward() always proceeds with the standard OAuth/cookie flow.

backend/internal/service/antigravity_gateway_service.go

@@ -1023,6 +1023,10 @@ func isModelNotFoundError(statusCode int, body []byte) bool {
 //	          ├─ 成功 → 正常返回
 //	          └─ 失败 → 设置模型限流 + 清除粘性绑定 → 切换账号
 func (s *AntigravityGatewayService) Forward(ctx context.Context, c *gin.Context, account *Account, body []byte, isStickySession bool) (*ForwardResult, error) {
+	if account.Type == AccountTypeUpstream {
+		return s.ForwardUpstream(ctx, c, account, body)
+	}
+
 	startTime := time.Now()
 
 	sessionID := getSessionID(c)