Fix SQL Server key-type migration issue and preserve Alembic revision

[behnam-o]

[This will require a force-push because it updates existing migration files]

This PR fixes a migration regression where Conversations.conversation_id could be treated as an unbounded string key type on SQL Server, which is invalid for indexed key columns.

Problem

• SQL Server rejects unbounded string key/index columns (e.g., VARCHAR/NVARCHAR(MAX)).
• The migration path causes failures during schema upgrade
• PR Fix integration test failures: Conversations table PK and stale attack_identifier references #2024 tries to solve it with an extra migration script, but that never runs because the problematic one (b2f4c6a8d1e3, being fixed here) is a prerequisite to that.

Solution

• Updated b2f4c6a8d1e3 to define Conversations.conversation_id as a bounded string (String(length=36)).
• Removed d4e5f6a7b8c9 revision which should no longer be needed.
• Added migration test coverage to catch unbounded string primary keys in revision files going forward.