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/service/billing_service.go

@@ -402,12 +402,6 @@ func (s *BillingService) GetModelPricingWithChannel(model string, channelPricing
 	return pricing, nil
 }
 
-// CalculateCostWithChannel 使用渠道定价计算费用
-// Deprecated: 使用 CalculateCostUnified 代替
-func (s *BillingService) CalculateCostWithChannel(model string, tokens UsageTokens, rateMultiplier float64, channelPricing *ChannelModelPricing) (*CostBreakdown, error) {
-	return s.calculateCostInternal(model, tokens, rateMultiplier, "", channelPricing)
-}
-
 // --- 统一计费入口 ---
 
 // CostInput 统一计费输入