assistant2: Keep the tool selector open when toggling tools (#26994)

This PR makes it so the tool selector will stay open when toggling tools
instead of closing after each selection:


https://github.com/user-attachments/assets/eb987785-cfb5-4b07-8d63-510fbd9d9bf1

This involved making a change to `ContextMenu` to allow it to rebuild
its menu items after each confirmation in order for them to reflect
their selected/unselected status. I intend to clean up the `ContextMenu`
API a bit at a later point, but that is out of scope for this PR.

Release Notes:

- N/A