Fix Alembic role migration truncation when renaming to worker_owner

[chrystianmartins]

Motivation

• Prevent a DataError: value too long for type character varying(8) when the migration updates users.role to worker_owner because the pre-migration enum/check constrained stored values to shorter length.
• Make the migration robust by widening the role column temporarily so data updates do not get truncated.

Description

• Updated pool-coordinator/alembic/versions/0003_update_user_role_enum_for_new_auth_flow.py to remove client and worker_owner from old_role_enum so it reflects the actual pre-migration enum (admin, operator, user).
• Added a temporary op.batch_alter_table in upgrade() that alters role to sa.String(length=32) before performing the UPDATE statements.
• Left the subsequent conversion to the new enum (client, worker_owner) intact so the column becomes the intended enum after data migration.

Testing

• Ran the test suite with cd pool-coordinator && python -m pytest -q, which completed successfully with 39 passed, 3 skipped.
• Executed the migration integration test with cd pool-coordinator && python -m pytest tests/integration/test_migrations_apply.py -q, which ran in this environment and reported 1 skipped.

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