feat(common): make ujust toggle-updates testable non-interactively

[castrojo]

The ujust toggle-updates recipe is currently tagged @pending in common_ujust.feature
because it presents an interactive menu (via gum) to confirm the toggle action.

The test cannot proceed past the prompt without user input, making automation impossible
with the current recipe implementation.

Options:

1. Add a --yes or --confirm flag to the toggle-updates recipe in projectbluefin/common
   that skips the confirmation prompt
2. Test the underlying systemd timer directly rather than via ujust:
   systemctl --user disable --now uupd.timer && systemctl --user enable --now uupd.timer
3. Use expect/pexpect in a test step to feed the confirmation automatically

The timer name may be uupd.timer or ublue-update.timer depending on image version.
Verify the correct timer name before implementing.

Tracking: common_ujust.feature toggle-updates scenario tagged @pending

[castrojo]

Blocked prerequisite: choosing a non-interactive ujust interface requires a projectbluefin/common contract change. The testsuite-only fallback is a direct timer-state assertion, but that changes the requested behavior and needs explicit acceptance before implementation.