bark: introduce locking on processing incoming mailbox messages

Concurrent consumers of mailbox messages could race and create duplicate movements (check test commit). We introduce a mailbox lock using the existing LockManager in bark.

Summary by CodeRabbit

• Bug Fixes
  • Prevented arkoor receives from being double-counted in wallet history when multiple mailbox sync consumers run concurrently for the same wallet.
  • Improved mailbox processing so checkpoints aren’t advanced when an arkoor message fails to process, ensuring consistent sync behavior.
• Tests
  • Added a Tokio concurrency test to verify overlapping mailbox sync operations result in exactly one recorded movement (and one final VTXO) for the pending arkoor receive.