assistant2: Cancel generation button (#23137)

Turns the "esc to cancel" label into a button so it can be dispatched
via click or keyboard. The keybinding isn't hardcoded anymore.

![CleanShot 2025-01-14 at 13 44
22@2x](https://github.com/user-attachments/assets/a947f58b-7de2-400b-b95a-384b78c79697)


Release Notes:

- N/A