feat: 完善使用记录端点可观测性与分布统计

将入站、上游与路径三类端点分布统一到使用记录页的一致化卡片交互中，并补齐端点元数据与统计链路，提升排障与流量分析效率。

backend/internal/handler/openai_gateway_endpoint_normalization_test.go

@@ -0,0 +1,57 @@
+package handler
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/require"
+)
+
+func TestNormalizedOpenAIUpstreamEndpoint(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	tests := []struct {
+		name     string
+		path     string
+		fallback string
+		want     string
+	}{
+		{
+			name:     "responses root maps to responses upstream",
+			path:     "/v1/responses",
+			fallback: openAIUpstreamEndpointResponses,
+			want:     "/v1/responses",
+		},
+		{
+			name:     "responses compact keeps compact suffix",
+			path:     "/openai/v1/responses/compact",
+			fallback: openAIUpstreamEndpointResponses,
+			want:     "/v1/responses/compact",
+		},
+		{
+			name:     "responses nested suffix preserved",
+			path:     "/openai/v1/responses/compact/detail",
+			fallback: openAIUpstreamEndpointResponses,
+			want:     "/v1/responses/compact/detail",
+		},
+		{
+			name:     "non responses path uses fallback",
+			path:     "/v1/messages",
+			fallback: openAIUpstreamEndpointResponses,
+			want:     "/v1/responses",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			rec := httptest.NewRecorder()
+			c, _ := gin.CreateTestContext(rec)
+			c.Request = httptest.NewRequest(http.MethodPost, tt.path, nil)
+
+			got := normalizedOpenAIUpstreamEndpoint(c, tt.fallback)
+			require.Equal(t, tt.want, got)
+		})
+	}
+}