Put back a check for IVsMonitorSelection to avoid use of UIContext

Right now it appears we have a bug that when that update runs,
it updates the Solution.WorkspaceVersion which breaks the ability
for TryApplyChanges to work later. CodeModel doesn't deal with that case
so leaving this in causes CodeModel tests to get flaky.

Furthermore, the WhenActivated() calls can't be unsubscribed, and will
leak for later tests, which can cause our tests to OOM in CI.

Author: jasonmalinowski

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

@@ -5,13 +5,20 @@
 using System.Linq;
 using Microsoft.CodeAnalysis.Host;
 using Microsoft.VisualStudio.Shell;
+using Microsoft.VisualStudio.Shell.Interop;
 
 namespace Microsoft.VisualStudio.LanguageServices.Implementation.ProjectSystem;
 
 internal abstract partial class VisualStudioWorkspaceImpl
 {
     private void SubscribeToSourceGeneratorImpactingEvents()
     {
+        // HACK: We don't want to subscribe to these in unit tests; the use of WhenActivated can cause a leak if these don't actually
+        // run, and that API gives us no way to unsubscribe. Further; right now raising events to update the source generator versions
+        // causes TryApplyChanges to also fail.
+        if (ServiceProvider.GlobalProvider.GetService(typeof(IVsMonitorSelection)) == null)
+            return;
+
         // This pattern ensures that we are called whenever the build starts/completes even if it is already in progress.
         KnownUIContexts.SolutionBuildingContext.WhenActivated(() =>
         {