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/service/channel_service.go

@@ -686,9 +686,7 @@ func (s *ChannelService) Create(ctx context.Context, input *CreateChannelInput)
 		ApplyPricingToAccountStats: input.ApplyPricingToAccountStats,
 		AccountStatsPricingRules:   input.AccountStatsPricingRules,
 	}
-	if channel.BillingModelSource == "" {
-		channel.BillingModelSource = BillingModelSourceChannelMapped
-	}
+	normalizeBillingModelSource(channel)
 
 	if err := validateChannelConfig(channel.ModelPricing, channel.ModelMapping); err != nil {
 		return nil, err
@@ -704,12 +702,31 @@ func (s *ChannelService) Create(ctx context.Context, input *CreateChannelInput)
 	}
 
 	s.invalidateCache()
-	return s.repo.GetByID(ctx, channel.ID)
+	created, err := s.repo.GetByID(ctx, channel.ID)
+	if err != nil {
+		return nil, err
+	}
+	normalizeBillingModelSource(created)
+	return created, nil
 }
 
-// GetByID 获取渠道详情
+// GetByID 获取渠道详情。返回前统一把空 BillingModelSource 回填为 ChannelMapped，
+// 让所有 handler 无需重复处理历史空值。
 func (s *ChannelService) GetByID(ctx context.Context, id int64) (*Channel, error) {
-	return s.repo.GetByID(ctx, id)
+	ch, err := s.repo.GetByID(ctx, id)
+	if err != nil {
+		return nil, err
+	}
+	normalizeBillingModelSource(ch)
+	return ch, nil
+}
+
+// normalizeBillingModelSource 若 BillingModelSource 为空则回填默认值 ChannelMapped。
+// 统一在 service 层完成，避免 handler 响应层重复兜底。
+func normalizeBillingModelSource(ch *Channel) {
+	if ch != nil && ch.BillingModelSource == "" {
+		ch.BillingModelSource = BillingModelSourceChannelMapped
+	}
 }
 
 // Update 更新渠道
@@ -741,7 +758,12 @@ func (s *ChannelService) Update(ctx context.Context, id int64, input *UpdateChan
 	s.invalidateCache()
 	s.invalidateAuthCacheForGroups(ctx, oldGroupIDs, channel.GroupIDs)
 
-	return s.repo.GetByID(ctx, id)
+	updated, err := s.repo.GetByID(ctx, id)
+	if err != nil {
+		return nil, err
+	}
+	normalizeBillingModelSource(updated)
+	return updated, nil
 }
 
 // applyUpdateInput 将更新请求的字段应用到渠道实体上。
@@ -859,7 +881,14 @@ func (s *ChannelService) Delete(ctx context.Context, id int64) error {
 
 // List 获取渠道列表
 func (s *ChannelService) List(ctx context.Context, params pagination.PaginationParams, status, search string) ([]Channel, *pagination.PaginationResult, error) {
-	return s.repo.List(ctx, params, status, search)
+	channels, res, err := s.repo.List(ctx, params, status, search)
+	if err != nil {
+		return nil, nil, err
+	}
+	for i := range channels {
+		normalizeBillingModelSource(&channels[i])
+	}
+	return channels, res, nil
 }
 
 // modelEntry 表示一个模型模式条目（用于冲突检测）