Revert some overzealous deleting.

Author: chsienki

src/VisualStudio/Core/Def/ProjectSystem/VisualStudioProjectFactory.cs

@@ -51,6 +51,23 @@ public VisualStudioProjectFactory(
         _dynamicFileInfoProviders = fileInfoProviders.AsImmutableOrEmpty();
         _analyzerAssemblyRedirectors = analyzerAssemblyRedirectors.AsImmutableOrEmpty();
         _solution = solution;
+
+        _threadingContext.JoinableTaskFactory.RunAsync(
+           async () =>
+           {
+               var cancellationToken = _threadingContext.DisposalToken;
+
+               // HACK: Fetch this service to ensure it's still created on the UI thread; once this is
+               // moved off we'll need to fix up it's constructor to be free-threaded.
+
+               // yield if on the main thread, as the VisualStudioMetadataReferenceManager construction can be fairly expensive
+               // and we don't want the case where VisualStudioProjectFactory is constructed on the main thread to block on that.
+               await _threadingContext.JoinableTaskFactory.SwitchToMainThreadAsync(alwaysYield: true, cancellationToken);
+               _visualStudioWorkspaceImpl.Services.GetRequiredService<VisualStudioMetadataReferenceManager>();
+
+               _visualStudioWorkspaceImpl.SubscribeExternalErrorDiagnosticUpdateSourceToSolutionBuildEvents();
+               _visualStudioWorkspaceImpl.SubscribeToSourceGeneratorImpactingEvents();
+           });
     }
 
     public Task<ProjectSystemProject> CreateAndAddToWorkspaceAsync(string projectSystemName, string language, CancellationToken cancellationToken)