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_upstream_context_test.go

@@ -0,0 +1,47 @@
+package service
+
+import (
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/require"
+)
+
+func TestAppendOpsUpstreamError_UsesRequestBodyBytesFromContext(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+
+	setOpsUpstreamRequestBody(c, []byte(`{"model":"gpt-5"}`))
+	appendOpsUpstreamError(c, OpsUpstreamErrorEvent{
+		Kind:    "http_error",
+		Message: "upstream failed",
+	})
+
+	v, ok := c.Get(OpsUpstreamErrorsKey)
+	require.True(t, ok)
+	events, ok := v.([]*OpsUpstreamErrorEvent)
+	require.True(t, ok)
+	require.Len(t, events, 1)
+	require.Equal(t, `{"model":"gpt-5"}`, events[0].UpstreamRequestBody)
+}
+
+func TestAppendOpsUpstreamError_UsesRequestBodyStringFromContext(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+
+	c.Set(OpsUpstreamRequestBodyKey, `{"model":"gpt-4"}`)
+	appendOpsUpstreamError(c, OpsUpstreamErrorEvent{
+		Kind:    "request_error",
+		Message: "dial timeout",
+	})
+
+	v, ok := c.Get(OpsUpstreamErrorsKey)
+	require.True(t, ok)
+	events, ok := v.([]*OpsUpstreamErrorEvent)
+	require.True(t, ok)
+	require.Len(t, events, 1)
+	require.Equal(t, `{"model":"gpt-4"}`, events[0].UpstreamRequestBody)
+}