Agents window Markdown editor: Rendered task checkboxes do not 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 `- [ ] task`.
2. Keep the task item inactive so it renders with a checkbox.
3. Click the checkbox.

### Expected

The checkbox toggles and the source changes from `[ ]` to `[x]`.

### Actual

The source remains `[ ]`. The task item enters raw editing mode and the rendered checkbox disappears.

### Impact

The control looks and behaves like an interactive checkbox until it is clicked, but it does not perform the checkbox action. This violates a strong UI affordance and makes a common task-list workflow require precise raw-marker editing instead of a single click.