test: validate database migration rollbacks

[afeezorobsco-cyber]

##close #1842

Summary

Adds test coverage validating database migration rollback behavior for the Mobile Money ↔ Stellar Bridge.

Changes

• Added migration apply/rollback tests.
• Verified schema changes before and after migrations.
• Verified rollback restores the expected previous schema.
• Covered migration ordering and rollback behavior.
• Covered data preservation/restoration where applicable.
• Uses an isolated test database/fixture.
• Added edge-case coverage for rollback behavior supported by the migration framework.

Testing

Covered:

• migration application
• migration rollback
• schema restoration
• data integrity
• multiple migration ordering
• latest-migration rollback
• re-application after rollback where supported

Validation

[PASTE MIGRATION TEST COMMAND]
[PASTE TEST COMMAND]
[PASTE LINT COMMAND]
[PASTE CLIPPY COMMAND]
[PASTE FORMAT COMMAND]

[drips-wave]

@afeezorobsco-cyber Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits

[afeezorobsco-cyber]

@sublime247 pls review and merge