[ML 0.5/MVI.02.3] Implement LocalMessageUidPrefixProvider

Description

Implement the LocalMessageUidPrefixProvider logic to retrieve the system-wide prefix used for messages that have not yet been synced with the server.

Goal

Decouple the deletion logic from legacy global constants (like K9.LOCAL_UID_PREFIX), allowing for cleaner testing and dependency injection.

Acceptance Criteria

• Correctly identifies messages generated locally that do not yet have a server-assigned UID.
• Replace direct calls from K9.LOCAL_UID_PREFIX to the newly created LocalMessageUidPrefixProvider.get()
• Testing: Unit tests verifying prefix detection across various message UID formats.