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_responses.go

@@ -80,6 +80,13 @@ 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
+	channelMapping, restricted := h.gatewayService.ResolveChannelMappingAndRestrict(c.Request.Context(), apiKey.GroupID, reqModel)
+	if restricted {
+		h.responsesErrorResponse(c, http.StatusServiceUnavailable, "api_error", "The requested model is not available for this API key")
+		return
+	}
+
 	// Claude Code only restriction:
 	// /v1/responses is never a Claude Code endpoint.
 	// When claude_code_only is enabled, this endpoint is rejected.
@@ -208,7 +215,11 @@ func (h *GatewayHandler) Responses(c *gin.Context) {
 
 		// 5. Forward request
 		writerSizeBeforeForward := c.Writer.Size()
-		result, err := h.gatewayService.ForwardAsResponses(c.Request.Context(), c, account, body, parsedReq)
+		forwardBody := body
+		if channelMapping.Mapped {
+			forwardBody = h.gatewayService.ReplaceModelInBody(body, channelMapping.MappedModel)
+		}
+		result, err := h.gatewayService.ForwardAsResponses(c.Request.Context(), c, account, forwardBody, parsedReq)
 
 		if accountReleaseFunc != nil {
 			accountReleaseFunc()
@@ -261,6 +272,10 @@ func (h *GatewayHandler) Responses(c *gin.Context) {
 				IPAddress:          clientIP,
 				RequestPayloadHash: requestPayloadHash,
 				APIKeyService:      h.apiKeyService,
+				ChannelID:          channelMapping.ChannelID,
+				OriginalModel:      reqModel,
+				BillingModelSource: channelMapping.BillingModelSource,
+				ModelMappingChain:  channelMapping.BuildModelMappingChain(reqModel, result.UpstreamModel),
 			}); err != nil {
 				reqLog.Error("gateway.responses.record_usage_failed",
 					zap.Int64("account_id", account.ID),