holdSet property
When set, setBackupConfirmed blocks on this until completed BEFORE it
mutates the flag — mirroring FakeWalletProvisioner.holdOpen. Lets a test
dispose the container WHILE a confirm/create-reset persist is in flight,
then release the gate and assert no post-dispose state = throws (the
_disposed guard after the await must swallow the continuation).
Implementation
Completer<void>? holdSet;