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/handler/gateway_handler.go

@@ -971,6 +971,10 @@ func (h *GatewayHandler) handleFailoverExhausted(c *gin.Context, failoverErr *se
 				msg = *rule.CustomMessage
 			}
 
+			if rule.SkipMonitoring {
+				c.Set(service.OpsSkipPassthroughKey, true)
+			}
+
 			h.handleStreamingAwareError(c, respCode, "upstream_error", msg, streamStarted)
 			return
 		}