- Expand migration comment to name the primary failure mode (missing
column OperationalError from #20842) ahead of the secondary SQLite
schema-reparse concern; also document the stale-cols-snapshot invariant
- Add clarifying comments on from_row() legacy fallback branches noting
they are belt-and-suspenders dead code post-migration
- Add task_events comment in existing test explaining why the table is
required by the migrator
- Add test_legacy_migration_no_legacy_columns_at_all: Scenario A —
explicitly asserts the exact #20842 crash no longer occurs and that
consecutive_failures defaults to 0 on a DB that never had spawn_failures
- Add test_legacy_migration_both_columns_already_present: Scenario D —
asserts the migration is a no-op when both columns already exist,
preserving the existing counter value
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