refactor(channels): centralize BillingModelSource normalization and exhaustive enum maps

- service: add normalizeBillingModelSource helper, apply in Create/GetByID/Update/List/ListAvailable outputs
- handler: drop channelToResponse fallback now that service owns the default; add passthrough test
- frontend: replace ternary status/billing-source lookups with Record<Enum, ...> maps so new union members fail the build
- chip/table: drop local type aliases, reuse UserSupportedModel/UserPricingInterval directly
- tests: assert short-circuit on ListAll error, wrap-prefix preservation, and Name-based default lookup

backend/internal/handler/admin/channel_handler.go

@@ -158,9 +158,6 @@ func channelToResponse(ch *service.Channel) *channelResponse {
 		UpdatedAt:      ch.UpdatedAt.Format("2006-01-02T15:04:05Z"),
 	}
 	resp.BillingModelSource = ch.BillingModelSource
-	if resp.BillingModelSource == "" {
-		resp.BillingModelSource = service.BillingModelSourceChannelMapped
-	}
 	if resp.GroupIDs == nil {
 		resp.GroupIDs = []int64{}
 	}

backend/internal/handler/admin/channel_handler_test.go

@@ -91,7 +91,7 @@ func TestChannelToResponse_EmptyDefaults(t *testing.T) {
 	ch := &service.Channel{
 		ID:                 1,
 		Name:               "ch",
-		BillingModelSource: "",
+		BillingModelSource: service.BillingModelSourceChannelMapped,
 		CreatedAt:          now,
 		UpdatedAt:          now,
 		GroupIDs:           nil,
@@ -105,6 +105,9 @@ func TestChannelToResponse_EmptyDefaults(t *testing.T) {
 		},
 	}
 
+	// handler 层 channelToResponse 现在是纯透传：BillingModelSource 的空值兜底
+	// 已下放到 service 层（Create/GetByID/List/Update/ListAvailable 出口统一处理），
+	// 因此这里构造 fixture 时直接传入归一化后的值。
 	resp := channelToResponse(ch)
 	require.Equal(t, "channel_mapped", resp.BillingModelSource)
 	require.NotNil(t, resp.GroupIDs)
@@ -117,6 +120,19 @@ func TestChannelToResponse_EmptyDefaults(t *testing.T) {
 	require.Equal(t, "token", resp.ModelPricing[0].BillingMode)
 }
 
+func TestChannelToResponse_BillingModelSourcePassthrough(t *testing.T) {
+	// handler 不再兜底 BillingModelSource：空值应原样透传（由 service 层负责默认回填）。
+	ch := &service.Channel{
+		ID:                 1,
+		Name:               "ch",
+		BillingModelSource: "",
+		CreatedAt:          time.Now(),
+		UpdatedAt:          time.Now(),
+	}
+	resp := channelToResponse(ch)
+	require.Equal(t, "", resp.BillingModelSource, "handler 应纯透传，默认值由 service.normalizeBillingModelSource 负责")
+}
+
 func TestChannelToResponse_NilModels(t *testing.T) {
 	now := time.Now()
 	ch := &service.Channel{