fix(openai): record zero-cost usage for unpriced models

backend/internal/service/billing_service.go

@@ -2,8 +2,8 @@ package service
 
 import (
 	"context"
+	"errors"
 	"fmt"
-
 	"log"
 	"strings"
 
@@ -110,6 +110,10 @@ type CostBreakdown struct {
 	BillingMode       string  // 计费模式（"token"/"per_request"/"image"），由 CalculateCostUnified 填充
 }
 
+// ErrModelPricingUnavailable indicates that none of the configured pricing
+// sources can price the requested model.
+var ErrModelPricingUnavailable = errors.New("pricing not found")
+
 // BillingService 计费服务
 type BillingService struct {
 	cfg            *config.Config
@@ -355,7 +359,7 @@ func (s *BillingService) GetModelPricing(model string) (*ModelPricing, error) {
 		return s.applyModelSpecificPricingPolicy(model, fallback), nil
 	}
 
-	return nil, fmt.Errorf("pricing not found for model: %s", model)
+	return nil, fmt.Errorf("%w for model: %s", ErrModelPricingUnavailable, model)
 }
 
 // GetModelPricingWithChannel 获取模型定价，渠道配置的价格覆盖默认值
@@ -452,7 +456,7 @@ func (s *BillingService) calculateTokenCost(resolved *ResolvedPricing, input Cos
 
 	pricing := input.Resolver.GetIntervalPricing(resolved, totalContext)
 	if pricing == nil {
-		return nil, fmt.Errorf("no pricing available for model: %s", input.Model)
+		return nil, fmt.Errorf("no pricing available for model: %s: %w", input.Model, ErrModelPricingUnavailable)
 	}
 
 	pricing = s.applyModelSpecificPricingPolicy(input.Model, pricing)