SOVD collapses prepare and execute terminal states into status=completed
and keeps the real phase on the vendor extension x-medkit-phase. Plugins
that split the install pipeline (uptane_ota and similar) emit
{ status: 'completed', 'x-medkit-phase': 'prepared' } after prepare -
the install still needs to be triggered. The old actionButtonsForStatus
only branched on status, so prepared updates flashed as fully terminal
and showed only Delete; the Execute button never rendered and users
had no UI path to finish the install.

Add the phase field to the UpdateStatus type and pass the full status
into actionButtonsForStatus. When phase === 'prepared' return
['execute', 'delete']; any other completed update is truly terminal
and keeps the previous ["delete"] behaviour. Existing tests still
pass; add three cases covering phase-missing, phase=prepared, and
phase=executed.

- Export UpdatePhase union (none/preparing/prepared/executing/executed/
  failed/deleting) matching the gateway `update_phase_to_string` enum
  and use it on UpdateStatus['x-medkit-phase'] instead of `string`. This
  gives compile-time checking for phase comparisons and typos.
- Add a default branch to actionButtonsForStatus so a non-conforming
  status string from a plugin falls back to `['delete']` instead of
  returning undefined and crashing the .map(...) render path.
- Cover the added edge cases in tests: unknown phase on completed
  (falls back to Delete-only) and phase on failed (ignored; retry
  actions still surface).