Handle validator register/unregister race with ordering guard

[GabrielePicco]

Tracking item from magicblock-validator PR #1314 review thread:
magicblock-labs/magicblock-validator#1314 (comment)

Problem:
During shutdown shortly after startup, startup registration may still be in flight while the validator starts unregistering. The unregister path can observe no registered validator yet and skip/return, then the earlier registration transaction can still confirm before exit. Since shutdown no longer performs a later unregister, the validator can remain registered on chain.

This should be guarded in the domain program so stale or out-of-order register/unregister transactions cannot leave stale validator state.

Possible direction:

• Add an ordering guard such as a nonce, epoch, or timestamp to validator registration state.
• Make unregister apply only to the intended current registration.
• Ensure an older in-flight registration cannot win after a newer unregister or superseding registration.

Acceptance criteria:

• Program logic prevents this stale registration outcome.
• Tests cover register-in-flight followed by shutdown/unregister ordering.
• Any required validator/client-side field changes are documented.