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/handler/openai_gateway_handler.go

@@ -1119,7 +1119,7 @@ func (h *OpenAIGatewayHandler) ResponsesWebSocket(c *gin.Context) {
 	setOpsRequestContext(c, reqModel, true, firstMessage)
 	setOpsEndpointContext(c, "", int16(service.RequestTypeWSV2))
 
-	// 解析渠道级模型映射 + 限制检查
+	// 解析渠道级模型映射
 	channelMappingWS, _ := h.gatewayService.ResolveChannelMappingAndRestrict(ctx, apiKey.GroupID, reqModel)
 
 	var currentUserRelease func()