Don't get IVsRunningDocumentTable on background thread

The `VisualStudioEditorDocumentManager` listens for running document
table events and was retrieving the `IVsRunningDocumentTable` in its
constructor. Because this is an `[ImportingConstructor]` it will
almost certain be called on the thread pool and will implicitly
marshal to the UI thread to perform a QI as part of retrieving the
service. This can cause a hang if the UI thread is tied up with other
work, such as when a solution is loaded.

This change avoids retrieving the running doc table service until its
needed and we are already running on the UI thread.

src/Razor/src/Microsoft.VisualStudio.LanguageServices.Razor/Documents/VisualStudioEditorDocumentManager.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.ComponentModel.Composition;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Runtime.InteropServices;
 using Microsoft.AspNetCore.Razor;
@@ -28,11 +29,12 @@ internal sealed class VisualStudioEditorDocumentManager(
     ProjectSnapshotManagerDispatcher dispatcher,
     JoinableTaskContext joinableTaskContext) : EditorDocumentManager(fileChangeTrackerFactory, dispatcher, joinableTaskContext)
 {
-    private readonly IVsRunningDocumentTable4 _runningDocumentTable = serviceProvider.GetService<SVsRunningDocumentTable, IVsRunningDocumentTable4>(throwOnFailure: true).AssumeNotNull();
+    private readonly IServiceProvider _serviceProvider = serviceProvider;
     private readonly IVsEditorAdaptersFactoryService _editorAdaptersFactory = editorAdaptersFactory;
 
     private readonly Dictionary<uint, List<DocumentKey>> _documentsByCookie = [];
     private readonly Dictionary<DocumentKey, uint> _cookiesByDocument = [];
+    private IVsRunningDocumentTable4? _runningDocumentTable;
     private bool _advised;
 
     protected override ITextBuffer? GetTextBufferForOpenDocument(string filePath)
@@ -219,10 +221,13 @@ internal sealed class VisualStudioEditorDocumentManager(
         }
     }
 
+    [MemberNotNull(nameof(_runningDocumentTable))]
     private void EnsureDocumentTableAdvised()
     {
         JoinableTaskContext.AssertUIThread();
 
+        _runningDocumentTable ??= _serviceProvider.GetService<SVsRunningDocumentTable, IVsRunningDocumentTable4>(throwOnFailure: true).AssumeNotNull();
+
         if (!_advised)
         {
             _advised = true;