style: 修复 gofmt 格式化问题

格式化以下测试文件以符合 Go 代码风格规范：
- fixtures_integration_test.go
- user_repo_integration_test.go
- api_key_service_delete_test.go

backend/internal/repository/fixtures_integration_test.go

@@ -390,4 +390,3 @@ func mustBindAccountToGroup(t *testing.T, client *dbent.Client, accountID, group
 		Save(ctx)
 	require.NoError(t, err, "create account_group")
 }
-

backend/internal/repository/user_repo_integration_test.go

@@ -507,4 +507,3 @@ func (s *UserRepoSuite) TestCRUD_And_Filters_And_AtomicUpdates() {
 	s.Require().Len(users, 1, "ListWithFilters len mismatch")
 	s.Require().Equal(user2.ID, users[0].ID, "ListWithFilters result mismatch")
 }
-