agent_ui: Fix keybinding conflict with action to clean up the queue (#47254)

This PR frees up `shift-backspace` to work as it did before. Now, the
action to remove the first queued message is now assigned to the
`cmd-shfit-backspace` keybinding, whereas the action to clear the entire
message queue is assigned to `cmd-alt-backspace`.

Release Notes:

- N/A
ff35da22fbDanilo Leal committed on 1/20/2026, 11:54:13 PM· committed by GitHubparentaa4e94f
3 files changedLine totals unavailable