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/gateway_handler_responses.go

@@ -80,7 +80,7 @@ func (h *GatewayHandler) Responses(c *gin.Context) {
 	setOpsRequestContext(c, reqModel, reqStream, body)
 	setOpsEndpointContext(c, "", int16(service.RequestTypeFromLegacy(reqStream, false)))
 
-	// 解析渠道级模型映射 + 限制检查
+	// 解析渠道级模型映射
 	channelMapping, _ := h.gatewayService.ResolveChannelMappingAndRestrict(c.Request.Context(), apiKey.GroupID, reqModel)
 
 	// Claude Code only restriction: