Fix ToString filename extraction for Windows paths

Agent-Logs-Url: https://github.com/PowerShell/PowerShellEditorServices/sessions/6285ab7f-3d36-4174-84cd-57dcebf32386

Co-authored-by: JustinGrote <15258962+JustinGrote@users.noreply.github.com>

Author: Copilot

src/PowerShellEditorServices/Extensions/EditorWorkspace.cs

@@ -33,7 +33,8 @@ internal EditorWorkspaceDocument(EditorWorkspace workspace, string path, bool sa
         /// <returns>The display name of this document.</returns>
         public override string ToString()
         {
-            string fileName = System.IO.Path.GetFileName(Path);
+            int fileNameStartIndex = System.Math.Max(Path.LastIndexOf('\\'), Path.LastIndexOf('/')) + 1;
+            string fileName = Path.Substring(fileNameStartIndex);
             return Saved ? fileName : fileName + " [Unsaved]";
         }