feat: support request_header key source (#4903)

* feat: support request_header key source in backend and settings UI

* feat: support request_header channel affinity source

service/channel_affinity_template_test.go

@@ -176,6 +176,66 @@ func TestShouldSkipRetryAfterChannelAffinityFailure(t *testing.T) {
 	}
 }
 
+func TestExtractChannelAffinityValue_RequestHeader(t *testing.T) {
+	rec := httptest.NewRecorder()
+	ctx, _ := gin.CreateTestContext(rec)
+	ctx.Request = httptest.NewRequest(http.MethodPost, "/v1/responses", nil)
+	ctx.Request.Header.Set("X-Affinity-Key", " tenant-123 ")
+
+	value := extractChannelAffinityValue(ctx, operation_setting.ChannelAffinityKeySource{
+		Type: "request_header",
+		Key:  "X-Affinity-Key",
+	})
+
+	require.Equal(t, "tenant-123", value)
+}
+
+func TestGetPreferredChannelByAffinity_RequestHeaderKeySource(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	rule := operation_setting.ChannelAffinityRule{
+		Name:       "header-affinity",
+		ModelRegex: []string{"^gpt-.*$"},
+		PathRegex:  []string{"/v1/responses"},
+		KeySources: []operation_setting.ChannelAffinityKeySource{
+			{Type: "request_header", Key: "X-Affinity-Key"},
+		},
+		IncludeRuleName:  true,
+		IncludeModelName: true,
+	}
+
+	affinityValue := fmt.Sprintf("header-hit-%d", time.Now().UnixNano())
+	cacheKeySuffix := buildChannelAffinityCacheKeySuffix(rule, "gpt-5", "default", affinityValue)
+
+	cache := getChannelAffinityCache()
+	require.NoError(t, cache.SetWithTTL(cacheKeySuffix, 9528, time.Minute))
+	t.Cleanup(func() {
+		_, _ = cache.DeleteMany([]string{cacheKeySuffix})
+	})
+
+	setting := operation_setting.GetChannelAffinitySetting()
+	originalRules := setting.Rules
+	setting.Rules = append([]operation_setting.ChannelAffinityRule{rule}, originalRules...)
+	t.Cleanup(func() {
+		setting.Rules = originalRules
+	})
+
+	rec := httptest.NewRecorder()
+	ctx, _ := gin.CreateTestContext(rec)
+	ctx.Request = httptest.NewRequest(http.MethodPost, "/v1/responses", nil)
+	ctx.Request.Header.Set("X-Affinity-Key", affinityValue)
+
+	channelID, found := GetPreferredChannelByAffinity(ctx, "gpt-5", "default")
+	require.True(t, found)
+	require.Equal(t, 9528, channelID)
+
+	meta, ok := getChannelAffinityMeta(ctx)
+	require.True(t, ok)
+	require.Equal(t, "request_header", meta.KeySourceType)
+	require.Equal(t, "X-Affinity-Key", meta.KeySourceKey)
+	require.Equal(t, buildChannelAffinityKeyHint(affinityValue), meta.KeyHint)
+}
+
 func TestChannelAffinityHitCodexTemplatePassHeadersEffective(t *testing.T) {
 	gin.SetMode(gin.TestMode)