Ensure the LSP server is loaded after a solution is closed (#79048)

Needed for dotnet/razor#9519

Previously Roslyn would load its language server when the first solution
is opened in VS so that workspace diagnostics and other such
functionality works. When the solution is closed however, VS would shut
down the server and the user has to manually open a `.cs` file to get it
to re-load. This ensures it is reloaded on solution open.

This is also needed to unblock cohosting, and cohosting integration
tests as well, as without this, when a 2nd solution is opened in VS the
user has to open a `.cs` file before they'll get any tooling in a
`.razor` file, because Razor uses dynamic registration which can only
happen after the LSP server has been loaded.

Author: davidwengier

src/EditorFeatures/Core/LanguageServer/AbstractInProcLanguageClient.cs

@@ -110,10 +110,16 @@ internal abstract partial class AbstractInProcLanguageClient(
 
     public event AsyncEventHandler<EventArgs>? StartAsync;
 
+    public event AsyncEventHandler<EventArgs>? StopAsync;
+
     /// <summary>
-    /// Unused, implementing <see cref="ILanguageClient"/>
+    /// Stops the server if it has been started.
     /// </summary>
-    public event AsyncEventHandler<EventArgs>? StopAsync { add { } remove { } }
+    /// <remarks>
+    /// Per the documentation on <see cref="ILanguageClient.StopAsync"/>, the event is ignored if the server has not been started.
+    /// </remarks>
+    public Task StopServerAsync()
+        => StopAsync?.InvokeAsync(this, EventArgs.Empty) ?? Task.CompletedTask;
 
     public async Task<Connection?> ActivateAsync(CancellationToken cancellationToken)
     {

src/EditorFeatures/Core/LanguageServer/AlwaysActiveLanguageClientEventListener.cs

@@ -40,8 +40,18 @@ internal sealed class AlwaysActiveLanguageClientEventListener(
     /// </summary>
     public void StartListening(Workspace workspace)
     {
-        // Trigger a fire and forget request to the VS LSP client to load our ILanguageClient.
-        Load();
+        _ = workspace.RegisterWorkspaceChangedHandler(Workspace_WorkspaceChanged);
+    }
+
+    private void Workspace_WorkspaceChanged(WorkspaceChangeEventArgs e)
+    {
+        if (e.Kind == WorkspaceChangeKind.SolutionAdded)
+        {
+            // Normally VS will load the language client when an editor window is created for one of our content types,
+            // but we want to load it as soon as a solution is loaded so workspace diagnostics work, and so 3rd parties
+            // like Razor can use dynamic registration.
+            Load();
+        }
     }
 
     public void StopListening(Workspace workspace)
@@ -56,12 +66,15 @@ private void Load()
 
         async Task LoadAsync()
         {
-
             // Explicitly switch to the bg so that if this causes any expensive work (like mef loads) it 
             // doesn't block the UI thread. Note, we always yield because sometimes our caller starts
             // on the threadpool thread but is indirectly blocked on by the UI thread.
             await TaskScheduler.Default.SwitchTo(alwaysYield: true);
 
+            // Sometimes the editor can be slow to stop the old server instance when the old solution is closed, so we force it here.
+            // This will no-op if the server hasn't been started yet.
+            await _languageClient.StopServerAsync().ConfigureAwait(false);
+
             await _languageClientBroker.Value.LoadAsync(new LanguageClientMetadata(
             [
                 ContentTypeNames.CSharpContentType,