Fix HasBeenDisposed accessibility for test access

Co-authored-by: QilongTang <3942418+QilongTang@users.noreply.github.com>

Author: Copilot

src/DynamoCore/Graph/ModelBase.cs

@@ -236,7 +236,7 @@ public virtual Guid GUID
         /// <summary>
         /// Has this <see cref="ModelBase"/> been disposed? Gets set when <see cref="Dispose"/> is called.
         /// </summary>
-        protected bool HasBeenDisposed { get; private set; }
+        protected internal bool HasBeenDisposed { get; private set; }
 
         /// <summary>
         /// Protected constructor.