Protect migration connection from direct commits

[rajvveer]

Summary

• add a MigrationCursor subclass that prevents migration code from retrieving its connection and committing directly
• pass the protected cursor to prerequisite, migration, and cleanup hooks while the framework retains its real cursor
• add a focused regression test for connection access

Validation

• Black formatting check passed
• Python compilation check passed
• standalone migration-cursor protection check passed
• the repository pytest harness requires its configured PostgreSQL service; the local run stopped at database authentication before collecting this test

Closes #3657

[hjanott]

For now we stall this PR since we are waiting for changes from feature/dammi which are necessary for this PR. It is important to understand that the migrations will have to be executed on a single transaction which is a part of feature/dammi.