feat(openai): 极致优化 OAuth 链路并补齐性能守护

- 优化 /v1/responses 热路径，减少重复解析与不必要拷贝\n- 优化并发与 token 竞争路径并补齐运行指标\n- 补充 OpenAI/Ops 相关单元测试与回归用例\n- 新增灰度阈值守护与压测脚本，支撑发布验收

backend/internal/service/ops_port.go

@@ -98,6 +98,10 @@ type OpsInsertErrorLogInput struct {
 	// It is set by OpsService.RecordError before persisting.
 	UpstreamErrorsJSON *string
 
+	AuthLatencyMs      *int64
+	RoutingLatencyMs   *int64
+	UpstreamLatencyMs  *int64
+	ResponseLatencyMs  *int64
 	TimeToFirstTokenMs *int64
 
 	RequestBodyJSON      *string // sanitized json string (not raw bytes)