fix: address review findings for channel restriction refactoring

- Fix 7 stale comments still mentioning "限制检查" in handlers/services
- Make billingModelForRestriction explicitly list channel_mapped case
- Add slog.Warn for error swallowing in ResolveChannelMapping and
  needsUpstreamChannelRestrictionCheck
- Document sticky session upstream check exemption

backend/internal/service/gateway_service.go

@@ -2967,6 +2967,8 @@ func (s *GatewayService) selectAccountForModelWithPlatform(ctx context.Context,
 	ctx = s.withRPMPrefetch(ctx, accounts)
 
 	// 3. 按优先级+最久未用选择（考虑模型支持）
+	// needsUpstreamCheck 仅在主选择循环中使用；粘性会话命中时跳过此检查，
+	// 因为粘性会话优先保持连接一致性，且 upstream 计费基准极少使用。
 	needsUpstreamCheck := s.needsUpstreamChannelRestrictionCheck(ctx, groupID)
 	var selected *Account
 	for i := range accounts {
@@ -3223,6 +3225,7 @@ func (s *GatewayService) selectAccountWithMixedScheduling(ctx context.Context, g
 	ctx = s.withRPMPrefetch(ctx, accounts)
 
 	// 3. 按优先级+最久未用选择（考虑模型支持和混合调度）
+	// needsUpstreamCheck 仅在主选择循环中使用；粘性会话命中时跳过此检查。
 	needsUpstreamCheck := s.needsUpstreamChannelRestrictionCheck(ctx, groupID)
 	var selected *Account
 	for i := range accounts {
@@ -8223,8 +8226,8 @@ func (s *GatewayService) IsModelRestricted(ctx context.Context, groupID int64, m
 	return s.channelService.IsModelRestricted(ctx, groupID, model)
 }
 
-// ResolveChannelMappingAndRestrict 解析渠道映射并检查模型限制。
-// 返回映射结果和是否被限制。
+// ResolveChannelMappingAndRestrict 解析渠道映射。
+// 模型限制检查已移至调度阶段（checkChannelPricingRestriction），restricted 始终返回 false。
 func (s *GatewayService) ResolveChannelMappingAndRestrict(ctx context.Context, groupID *int64, model string) (ChannelMappingResult, bool) {
 	if s.channelService == nil {
 		return ChannelMappingResult{MappedModel: model}, false
@@ -8255,7 +8258,9 @@ func billingModelForRestriction(source, requestedModel, channelMappedModel strin
 		return requestedModel
 	case BillingModelSourceUpstream:
 		return ""
-	default: // channel_mapped
+	case BillingModelSourceChannelMapped:
+		return channelMappedModel
+	default:
 		return channelMappedModel
 	}
 }
@@ -8287,7 +8292,11 @@ func (s *GatewayService) needsUpstreamChannelRestrictionCheck(ctx context.Contex
 		return false
 	}
 	ch, err := s.channelService.GetChannelForGroup(ctx, *groupID)
-	if err != nil || ch == nil || !ch.RestrictModels {
+	if err != nil {
+		slog.Warn("failed to check channel upstream restriction", "group_id", *groupID, "error", err)
+		return false
+	}
+	if ch == nil || !ch.RestrictModels {
 		return false
 	}
 	return ch.BillingModelSource == BillingModelSourceUpstream