fix: keep untitled buffers discoverable

Allow TryGetFile() to return existing in-memory untitled documents.
This preserves the debugger's untitled script workflow while still letting
provider-backed paths flow through WorkspaceService.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: dkattan

src/PowerShellEditorServices/Services/Workspace/WorkspaceService.cs

@@ -195,7 +195,7 @@ public bool TryGetFile(Uri fileUri, out ScriptFile scriptFile) =>
         /// <param name="scriptFile">The out parameter that will contain the ScriptFile object.</param>
         public bool TryGetFile(DocumentUri documentUri, out ScriptFile scriptFile)
         {
-            if (ScriptFile.IsUntitledPath(documentUri.ToString()) || !ScriptFile.IsSupportedScheme(documentUri.Scheme))
+            if (!ScriptFile.IsSupportedScheme(documentUri.Scheme))
             {
                 scriptFile = null;
                 return false;