Harden ToString path handling for null and separators

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

@@ -33,8 +33,10 @@ internal EditorWorkspaceDocument(EditorWorkspace workspace, string path, bool sa
         /// <returns>The display name of this document.</returns>
         public override string ToString()
         {
-            int fileNameStartIndex = System.Math.Max(Path.LastIndexOf('\\'), Path.LastIndexOf('/')) + 1;
-            string fileName = Path.Substring(fileNameStartIndex);
+            string documentPath = Path ?? string.Empty;
+            // Handle Windows and POSIX separators consistently across platforms.
+            int fileNameStartIndex = System.Math.Max(documentPath.LastIndexOf('\\'), documentPath.LastIndexOf('/')) + 1;
+            string fileName = documentPath.Substring(fileNameStartIndex);
             return Saved ? fileName : fileName + " [Unsaved]";
         }