apply some suggested minor fixes around debugger helpers

Author: hahn-kev

backend/FwLite/LcmDebugger/FakeSyncSource.cs

@@ -50,7 +50,8 @@ public static FakeSyncSource FromJsonFile(string path, JsonSerializerOptions? op
             };
         }
 
-        var changes = JsonSerializer.Deserialize<ChangesResult<Commit>>(File.OpenRead(path), options);
+        using var file = File.OpenRead(path);
+        var changes = JsonSerializer.Deserialize<ChangesResult<Commit>>(file, options);
         ArgumentNullException.ThrowIfNull(changes);
         ArgumentNullException.ThrowIfNull(changes.MissingFromClient);
         ArgumentNullException.ThrowIfNull(changes.ServerSyncState);

backend/FwLite/LcmDebugger/Utils.cs

@@ -52,7 +52,9 @@ public static async Task<CrdtMiniLcmApi> NewProjectFromSyncable(this IServicePro
         var crdtProjectsService = services.GetRequiredService<CrdtProjectsService>();
         var crdtProject = await crdtProjectsService.CreateProject(new CrdtProjectsService.CreateProjectRequest("test-project", $"test-{Guid.NewGuid().ToString().Split('-')[0]}", projectId));
         var crdtMiniLcmApi = (CrdtMiniLcmApi)await crdtProjectsService.OpenProject(crdtProject, services);
-        await services.GetRequiredService<DataModel>().SyncWith(syncable);
+        var syncResult = await services.GetRequiredService<DataModel>().SyncWith(syncable);
+        if (!syncResult.IsSynced)
+            throw new InvalidOperationException("New project sync failed.");
         return crdtMiniLcmApi;
     }