Reverts 525e36366 (graceful legacy-cursor handling). The legacy bare-id cursor
only matters for a backfill in flight across the single deploy that changed the
cursor format, and that backfill is internal and admin-triggered, so the
one-time transition is handled operationally: do not run a backfill during the
rollout deploy. Keeping decodeBackfillCursor throwing surfaces genuine cursor
corruption loudly instead of silently restarting the window. The old keyset was
id-ordered and the new one createdAt-ordered, so a stale cursor cannot be safely
resumed anyway, only restarted.
Dan Sutton
3418d9d0a7