[eersnington]

• Fixes a regression from fix(worker): stop alarms on migrated durable objects #2: an object exported a second time reported no alarm, because the first export had already removed the Cloudflare alarm. The export now falls back to the alarm time saved when the object was marked migrated.
• This happens whenever the destination fails after the last page and migrate runs again.
• Adds a test that exports the same object twice and checks the alarm is in both copies.
• Adds a smoke check that a migrated object exported again on real Cloudflare still reports its alarm.