Every round-trip test across test_book_migration.py, test_scribe_migration.py
and test_tenants_migration.py re-upgraded to its own revision on the way back
up rather than to head, so a full suite run left core_api_test one or two
migrations behind (008, with 009/010 released since). Each now ends on
`upgrade head`; a new TestSchemaEndsAtHeadAfterTheMigrationSuite class checks
the final state directly with no upgrade of its own, as a regression guard.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>