fix(billing): prevent channel_mapped override from reverting BillingModel when channel did not map

When a channel has no model mapping for the requested model, ChannelMappedModel
equals OriginalModel (the user's arbitrary input). Combined with the default
BillingModelSource="channel_mapped", this incorrectly overrides the BillingModel
set by the OpenAI format conversion layer (e.g., gpt-5.4 from DefaultMappedModel)
back to the unmapped original model (e.g., glm) which has no pricing — resulting
in zero-cost billing.

Add guard condition so the channel_mapped override only fires when the channel
actually changed the model (ChannelMappedModel != OriginalModel).

backend/internal/service/openai_gateway_record_usage_test.go

@@ -933,6 +933,89 @@ func TestOpenAIGatewayServiceRecordUsage_BillsMappedRequestsUsingRequestedModel(
 	require.Equal(t, expectedCost.ActualCost, userRepo.lastAmount)
 }
 
+func TestOpenAIGatewayServiceRecordUsage_ChannelMappedDoesNotOverrideBillingModelWhenUnmapped(t *testing.T) {
+	usageRepo := &openAIRecordUsageLogRepoStub{inserted: true}
+	userRepo := &openAIRecordUsageUserRepoStub{}
+	subRepo := &openAIRecordUsageSubRepoStub{}
+	svc := newOpenAIRecordUsageServiceForTest(usageRepo, userRepo, subRepo, nil)
+	usage := OpenAIUsage{InputTokens: 20, OutputTokens: 10}
+
+	// When channel did NOT map the model (ChannelMappedModel == OriginalModel),
+	// billing should use result.BillingModel (the actual model used after group
+	// DefaultMappedModel resolution), not the unmapped original model.
+	expectedCost, err := svc.billingService.CalculateCost("gpt-5.1", UsageTokens{
+		InputTokens:  20,
+		OutputTokens: 10,
+	}, 1.1)
+	require.NoError(t, err)
+
+	err = svc.RecordUsage(context.Background(), &OpenAIRecordUsageInput{
+		Result: &OpenAIForwardResult{
+			RequestID:     "resp_channel_unmapped_billing",
+			Model:         "glm",
+			BillingModel:  "gpt-5.1",
+			UpstreamModel: "gpt-5.1",
+			Usage:         usage,
+			Duration:      time.Second,
+		},
+		APIKey:  &APIKey{ID: 10},
+		User:    &User{ID: 20},
+		Account: &Account{ID: 30},
+		ChannelUsageFields: ChannelUsageFields{
+			ChannelID:          1,
+			OriginalModel:      "glm",
+			ChannelMappedModel: "glm", // channel did NOT map
+			BillingModelSource: BillingModelSourceChannelMapped,
+		},
+	})
+
+	require.NoError(t, err)
+	require.NotNil(t, usageRepo.lastLog)
+	require.Equal(t, expectedCost.ActualCost, usageRepo.lastLog.ActualCost)
+	require.True(t, usageRepo.lastLog.ActualCost > 0, "cost must not be zero")
+}
+
+func TestOpenAIGatewayServiceRecordUsage_ChannelMappedOverridesBillingModelWhenMapped(t *testing.T) {
+	usageRepo := &openAIRecordUsageLogRepoStub{inserted: true}
+	userRepo := &openAIRecordUsageUserRepoStub{}
+	subRepo := &openAIRecordUsageSubRepoStub{}
+	svc := newOpenAIRecordUsageServiceForTest(usageRepo, userRepo, subRepo, nil)
+	usage := OpenAIUsage{InputTokens: 20, OutputTokens: 10}
+
+	// When channel DID map the model (ChannelMappedModel != OriginalModel),
+	// billing should use the channel-mapped model, honoring admin intent.
+	expectedCost, err := svc.billingService.CalculateCost("gpt-5.1", UsageTokens{
+		InputTokens:  20,
+		OutputTokens: 10,
+	}, 1.1)
+	require.NoError(t, err)
+
+	err = svc.RecordUsage(context.Background(), &OpenAIRecordUsageInput{
+		Result: &OpenAIForwardResult{
+			RequestID:     "resp_channel_mapped_billing",
+			Model:         "glm",
+			BillingModel:  "gpt-5.1-codex",
+			UpstreamModel: "gpt-5.1-codex",
+			Usage:         usage,
+			Duration:      time.Second,
+		},
+		APIKey:  &APIKey{ID: 10},
+		User:    &User{ID: 20},
+		Account: &Account{ID: 30},
+		ChannelUsageFields: ChannelUsageFields{
+			ChannelID:          1,
+			OriginalModel:      "glm",
+			ChannelMappedModel: "gpt-5.1", // channel mapped glm → gpt-5.1
+			BillingModelSource: BillingModelSourceChannelMapped,
+		},
+	})
+
+	require.NoError(t, err)
+	require.NotNil(t, usageRepo.lastLog)
+	require.Equal(t, expectedCost.ActualCost, usageRepo.lastLog.ActualCost)
+	require.True(t, usageRepo.lastLog.ActualCost > 0, "cost must not be zero")
+}
+
 func TestOpenAIGatewayServiceRecordUsage_SubscriptionBillingSetsSubscriptionFields(t *testing.T) {
 	usageRepo := &openAIRecordUsageLogRepoStub{inserted: true}
 	userRepo := &openAIRecordUsageUserRepoStub{}

backend/internal/service/openai_gateway_service.go

@@ -4277,7 +4277,7 @@ func (s *OpenAIGatewayService) RecordUsage(ctx context.Context, input *OpenAIRec
 	if result.BillingModel != "" {
 		billingModel = strings.TrimSpace(result.BillingModel)
 	}
-	if input.BillingModelSource == BillingModelSourceChannelMapped && input.ChannelMappedModel != "" {
+	if input.BillingModelSource == BillingModelSourceChannelMapped && input.ChannelMappedModel != "" && input.ChannelMappedModel != input.OriginalModel {
 		billingModel = input.ChannelMappedModel
 	}
 	if input.BillingModelSource == BillingModelSourceRequested && input.OriginalModel != "" {