fix: skip rate limiting when custom error codes don't match upstream status

Add ShouldHandleErrorCode guard at the entry of handleGeminiUpstreamError
and AntigravityGatewayService.handleUpstreamError so that accounts with
custom error codes (e.g. [599]) are not rate-limited when the upstream
returns a non-matching status (e.g. 429).

backend/internal/service/antigravity_gateway_service.go

@@ -2243,6 +2243,10 @@ func (s *AntigravityGatewayService) handleUpstreamError(
 	requestedModel string,
 	groupID int64, sessionHash string, isStickySession bool,
 ) *handleModelRateLimitResult {
+	// 遵守自定义错误码策略：未命中则跳过所有限流处理
+	if !account.ShouldHandleErrorCode(statusCode) {
+		return nil
+	}
 	// 模型级限流处理（优先）
 	result := s.handleModelRateLimit(&handleModelRateLimitParams{
 		ctx:             ctx,