fix: 修复错误透传规则 skip_monitoring 未生效的问题

- ops_error_logger: status < 400 分支增加 OpsSkipPassthroughKey 检查
- ops_upstream_context: 新增 checkSkipMonitoringForUpstreamEvent，中间重试/故障转移事件也能触发跳过标记
- gateway_handler/openai_gateway_handler/gemini_v1beta_handler: handleFailoverExhausted 匹配规则后设置 OpsSkipPassthroughKey
- antigravity_gateway_service: writeMappedClaudeError 增加 applyErrorPassthroughRule 调用

backend/internal/service/antigravity_gateway_service.go

@@ -3251,6 +3251,21 @@ func (s *AntigravityGatewayService) writeMappedClaudeError(c *gin.Context, accou
 		log.Printf("[antigravity-Forward] upstream_error status=%d body=%s", upstreamStatus, truncateForLog(body, maxBytes))
 	}
 
+	// 检查错误透传规则
+	if ptStatus, ptErrType, ptErrMsg, matched := applyErrorPassthroughRule(
+		c, account.Platform, upstreamStatus, body,
+		0, "", "",
+	); matched {
+		c.JSON(ptStatus, gin.H{
+			"type":  "error",
+			"error": gin.H{"type": ptErrType, "message": ptErrMsg},
+		})
+		if upstreamMsg == "" {
+			return fmt.Errorf("upstream error: %d", upstreamStatus)
+		}
+		return fmt.Errorf("upstream error: %d message=%s", upstreamStatus, upstreamMsg)
+	}
+
 	var statusCode int
 	var errType, errMsg string