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/ops_error_logger.go

@@ -537,6 +537,13 @@ func OpsErrorLoggerMiddleware(ops *service.OpsService) gin.HandlerFunc {
 			// Store request headers/body only when an upstream error occurred to keep overhead minimal.
 			entry.RequestHeadersJSON = extractOpsRetryRequestHeaders(c)
 
+			// Skip logging if a passthrough rule with skip_monitoring=true matched.
+			if v, ok := c.Get(service.OpsSkipPassthroughKey); ok {
+				if skip, _ := v.(bool); skip {
+					return
+				}
+			}
+
 			enqueueOpsErrorLog(ops, entry, requestBody)
 			return
 		}