Agents window Markdown editor: Home and End ignore visual line wrapping

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 with a long paragraph.
2. Make the editor narrow enough that the paragraph wraps to at least three visual lines.
3. Put the caret on the middle visual line.
4. Press Home, then repeat and press End.

### Expected

To match the normal VS Code editor, Home and End move to the start and end of the current visual line.

### Actual

Home moves to the beginning of the entire Markdown paragraph, while End moves to the end of the entire paragraph.

### Impact

Editing long prose becomes inefficient because users cannot quickly reach a wrapped line boundary. Large caret jumps are also disorienting and make selection with Shift+Home/End difficult to control.