Agents window Markdown editor: Support standard bold and italic shortcuts

Testing #326540

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

### Scenario

1. Open a Markdown document in **Editing** mode.
2. Select a word.
3. Press Cmd+B or Cmd+I.

### Current Behavior

Neither shortcut changes the selected text.

### Requested Behavior

- Cmd+B wraps or unwraps the selection with strong-emphasis markers.
- Cmd+I wraps or unwraps the selection with emphasis markers.
- Windows/Linux should provide the corresponding Ctrl+B and Ctrl+I commands.

### Impact

These are established rich-text and Markdown editing shortcuts. Their absence makes common formatting slower, forces users to manage delimiter pairs manually, and makes the hybrid editor feel less capable than both the normal text editor and typical Markdown editors.