fix(billing): 计费始终使用用户请求的原始模型，而非映射后的上游模型

当账号配置了模型映射（如 claude-sonnet-4-6 → glm-5.0）时，系统错误地
使用映射后的上游模型名计算费用。由于上游模型（如 glm-5.0）在定价系统中
没有价格配置，导致计费失败后被静默置为 0，用户不被扣费。

修改 forwardResultBillingModel 优先返回请求模型名，并移除 OpenAI 路径
中 BillingModel 字段对计费模型的覆盖逻辑。

backend/internal/service/openai_gateway_record_usage_test.go

@@ -895,14 +895,16 @@ func TestOpenAIGatewayServiceRecordUsage_UsesRequestedModelAndUpstreamModelMetad
 	require.Equal(t, 1, userRepo.deductCalls)
 }
 
-func TestOpenAIGatewayServiceRecordUsage_BillsMappedRequestsUsingUpstreamModelFallback(t *testing.T) {
+func TestOpenAIGatewayServiceRecordUsage_BillsMappedRequestsUsingRequestedModel(t *testing.T) {
 	usageRepo := &openAIRecordUsageLogRepoStub{inserted: true}
 	userRepo := &openAIRecordUsageUserRepoStub{}
 	subRepo := &openAIRecordUsageSubRepoStub{}
 	svc := newOpenAIRecordUsageServiceForTest(usageRepo, userRepo, subRepo, nil)
 	usage := OpenAIUsage{InputTokens: 20, OutputTokens: 10}
 
-	expectedCost, err := svc.billingService.CalculateCost("gpt-5.1-codex", UsageTokens{
+	// Billing should use the requested model ("gpt-5.1"), not the upstream mapped model ("gpt-5.1-codex").
+	// This ensures pricing is always based on the model the user requested.
+	expectedCost, err := svc.billingService.CalculateCost("gpt-5.1", UsageTokens{
 		InputTokens:  20,
 		OutputTokens: 10,
 	}, 1.1)

backend/internal/service/openai_gateway_service.go

@@ -4153,9 +4153,6 @@ func (s *OpenAIGatewayService) RecordUsage(ctx context.Context, input *OpenAIRec
 	}
 
 	billingModel := forwardResultBillingModel(result.Model, result.UpstreamModel)
-	if result.BillingModel != "" {
-		billingModel = strings.TrimSpace(result.BillingModel)
-	}
 	serviceTier := ""
 	if result.ServiceTier != nil {
 		serviceTier = strings.TrimSpace(*result.ServiceTier)

backend/internal/service/usage_log_helpers.go

@@ -21,8 +21,8 @@ func optionalNonEqualStringPtr(value, compare string) *string {
 }
 
 func forwardResultBillingModel(requestedModel, upstreamModel string) string {
-	if trimmedUpstream := strings.TrimSpace(upstreamModel); trimmedUpstream != "" {
-		return trimmedUpstream
+	if trimmed := strings.TrimSpace(requestedModel); trimmed != "" {
+		return trimmed
 	}
-	return strings.TrimSpace(requestedModel)
+	return strings.TrimSpace(upstreamModel)
 }