fix(channel): 全平台渠道映射覆盖 + 公共函数抽取 + 死代码清理

- 4个缺失handler入口添加渠道映射+限制检查(ChatCompletions/Responses/Gemini)
- 模型限制错误信息优化，区分"模型不可用"和"无账号"
- OpenAI RecordUsage RequestedModel 改用 OriginalModel
- ResolveChannelMappingAndRestrict/ReplaceModelInBody 抽取到 ChannelService 消除跨service重复
- validateNoDuplicateModels 按 platform:model 去重
- 删除 Channel.ResolveMappedModel 死代码和 CalculateCostWithChannel Deprecated方法
- 移除冗余nil检查，抽取 validatePricingBillingMode 公共校验

backend/internal/handler/gateway_handler.go

@@ -161,7 +161,7 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 	// 解析渠道级模型映射 + 限制检查
 	channelMapping, restricted := h.gatewayService.ResolveChannelMappingAndRestrict(c.Request.Context(), apiKey.GroupID, reqModel)
 	if restricted {
-		h.errorResponse(c, http.StatusServiceUnavailable, "api_error", "No available accounts")
+		h.errorResponse(c, http.StatusServiceUnavailable, "api_error", "The requested model is not available for this API key")
 		return
 	}