Agents window Markdown editor: Tab in a list item focuses the lock toggle

Testing #326540

Does this issue occur when all extensions are disabled?: Yes

- VS Code Version: Insiders with the experimental Markdown editor
- OS Version: macOS
- Audited package revision: microsoft/vscode-packages@1d81418d61ad769f154579196a51a9364c105e2d

### Steps to Reproduce

1. Open a Markdown document containing `- first`.
2. Put the caret in the list item while in **Editing** mode.
3. Press Tab.

### Expected

Tab indents the list item, consistent with normal Markdown editing behavior in VS Code.

### Actual

Focus leaves the document and moves to the Editing/Locked toggle.

### Impact

Keyboard users cannot structure nested lists with the standard command. Focus also lands on a mode-changing control, so a subsequent Enter or Space can accidentally lock the editor. The unexpected focus change interrupts typing and is difficult to notice when focus styling is subtle.