Extends the previous commit to cover the remaining additive-column index
that sits on the same migration trap:
- ``task_events.run_id`` -> ``idx_events_run`` was still in SCHEMA_SQL.
A legacy ``task_events`` table predating #17805 (no ``run_id``) would
still abort ``executescript`` before ``_migrate_add_optional_columns``
could add the column. Hoisted out of SCHEMA_SQL and made unconditional
in the migration alongside the other three indexes.
- Removed the now-redundant ``CREATE INDEX idx_tasks_idempotency`` that
was nested inside the ``if "idempotency_key" not in cols`` branch.
The unconditional create lower in the function makes it idempotent
on both fresh and legacy DBs.
- Strengthened the regression test to cover all four indexes
(``idx_tasks_session_id``, ``idx_tasks_tenant``, ``idx_tasks_idempotency``,
``idx_events_run``) and to seed a pre-#17805 ``task_events`` shape that
exercises the ``run_id`` migration path.
The result: every ``CREATE INDEX`` that depends on an additive column now
runs after the migration ensures the column exists. Verified against a
realistic pre-#16081 board fixture (tasks + task_events both legacy
shape) — origin/main reproduces ``no such column: session_id``; this
branch migrates cleanly and creates all four indexes.
7552e0f3c0