The _column_exists() helper in migrations 026-029 calls conn.execute()
which returns None in Alembic's offline/SQL mode (--sql flag), causing
AttributeError on result.scalar(). Add context.is_offline_mode() guard
to skip the idempotency check in offline mode and unconditionally emit
the DDL statements.
Fixes: Validate CI step 'alembic upgrade head --sql' failure.