feat(billing): 网关计费迁移到 CalculateCostUnified + 模型限制错误统一

- GatewayService/OpenAIGatewayService 注入 ModelPricingResolver
- RecordUsage 从旧路径迁移到 CalculateCostUnified（支持 per_request/image 模式）
- 无渠道时自动回退旧路径，保持原有行为
- 长上下文双倍计费仅在无渠道定价时生效
- CostBreakdown 新增 BillingMode 字段，使用日志记录实际计费模式
- 模型限制错误改为与"无可用账号"相同的 503 响应

backend/internal/handler/gateway_handler.go

@@ -164,14 +164,10 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 		channelMapping = h.gatewayService.ResolveChannelMapping(c.Request.Context(), *apiKey.GroupID, reqModel)
 	}
 
-	// 渠道模型限制检查
+	// 渠道模型限制检查：使用原始请求模型名，因为定价列表中注册的是用户请求的模型名
 	if apiKey.GroupID != nil {
-		checkModel := reqModel
-		if channelMapping.Mapped {
-			checkModel = channelMapping.MappedModel
-		}
-		if h.gatewayService.IsModelRestricted(c.Request.Context(), *apiKey.GroupID, checkModel) {
-			h.errorResponse(c, http.StatusForbidden, "invalid_request_error", "Model not available in current channel: "+reqModel)
+		if h.gatewayService.IsModelRestricted(c.Request.Context(), *apiKey.GroupID, reqModel) {
+			h.errorResponse(c, http.StatusServiceUnavailable, "api_error", "No available accounts")
 			return
 		}
 	}

backend/internal/handler/gateway_handler_warmup_intercept_unit_test.go

@@ -162,6 +162,7 @@ func newTestGatewayHandler(t *testing.T, group *service.Group, accounts []*servi
 		nil, // settingService
 		nil, // tlsFPProfileService
 		nil, // channelService
+		nil, // resolver
 	)
 
 	// RunModeSimple：跳过计费检查，避免引入 repo/cache 依赖。

backend/internal/handler/sora_client_handler_test.go

@@ -2224,7 +2224,8 @@ func (s *stubSoraClientForHandler) GetVideoTask(_ context.Context, _ *service.Ac
 func newMinimalGatewayService(accountRepo service.AccountRepository) *service.GatewayService {
 	return service.NewGatewayService(
 		accountRepo, nil, nil, nil, nil, nil, nil, nil, nil,
-		nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil,
+		nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil,
+		nil, nil,
 	)
 }
 

backend/internal/handler/sora_gateway_handler_test.go

@@ -466,6 +466,7 @@ func TestSoraGatewayHandler_ChatCompletions(t *testing.T) {
 		nil, // settingService
 		nil, // tlsFPProfileService
 		nil, // channelService
+		nil, // resolver
 	)
 
 	soraClient := &stubSoraClient{imageURLs: []string{"https://example.com/a.png"}}