Assert the file is open in Workspace.OnAdditionalDocumentClosed

Previously opened additional documents weren't counted as open in
Workspace.IsDocumentOpen, but after
https://github.com/dotnet/roslyn/pull/33165 we can now properly do this
check.

src/Workspaces/Core/Portable/Workspace/Workspace_Editor.cs

@@ -556,6 +556,7 @@ protected internal void OnAdditionalDocumentClosed(DocumentId documentId, TextLo
             using (_serializationLock.DisposableWait())
             {
                 this.CheckAdditionalDocumentIsInCurrentSolution(documentId);
+                this.CheckDocumentIsOpen(documentId);
 
                 // forget any open document info
                 ClearOpenDocument(documentId);