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/gemini_v1beta_handler.go

@@ -184,7 +184,7 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 	setOpsRequestContext(c, modelName, stream, body)
 	setOpsEndpointContext(c, "", int16(service.RequestTypeFromLegacy(stream, 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, modelName)
 	reqModel := modelName // 保存映射前的原始模型名
 	if channelMapping.Mapped {