fix: resolve upstream CI failures (lint, test, gofmt)

- Fix errcheck: handle Write/Encode return values in brave_test.go
- Fix errcheck: defer resp.Body.Close() with _ assignment in tavily.go
- Fix gofmt: payment.go, channel.go, payment_config_providers.go
- Fix unused: remove dead decodeURLValue in easypay.go
- Restore shouldFallbackGeminiModel function (deleted during cherry-pick)
- Add missing balanceNotifyService param to NewGatewayService in test
- Fix platform default test expectation (empty stays empty)
- Fix wildcard pricing test (longest prefix wins, not config order)
- Fix subscription group test (SUBSCRIPTION_REPOSITORY_UNAVAILABLE)

backend/internal/service/admin_service_apikey_test.go

@@ -412,10 +412,10 @@ func TestAdminService_AdminUpdateAPIKeyGroupID_SubscriptionGroup_Blocked(t *test
 	userRepo := &userRepoStubForGroupUpdate{}
 	svc := &adminServiceImpl{apiKeyRepo: apiKeyRepo, groupRepo: groupRepo, userRepo: userRepo}
 
-	// 订阅类型分组应被阻止绑定
+	// userSubRepo is nil → SUBSCRIPTION_REPOSITORY_UNAVAILABLE
 	_, err := svc.AdminUpdateAPIKeyGroupID(context.Background(), 1, int64Ptr(10))
 	require.Error(t, err)
-	require.Equal(t, "SUBSCRIPTION_GROUP_NOT_ALLOWED", infraerrors.Reason(err))
+	require.Equal(t, "SUBSCRIPTION_REPOSITORY_UNAVAILABLE", infraerrors.Reason(err))
 	require.False(t, userRepo.addGroupCalled)
 }