sql: Check new PTS behavior for backfills

This issue tracks the work to run a manual test triggering a large index backfill operation that would have needed the use of PTS previously (i.e. an index backfill that lasts longer than the GC TTL duration) and compare the difference between the old backfill code that uses PTS vs. the new version that doesn’t.


Jira issue: CRDB-49443