test(shared): cover an outside reader stopping TRUNCATE on a backfilled WAL

The pinned-reader test now also opens the reader after 38 writes, once the WAL
is fully backfilled, so the reader's snapshot stops the TRUNCATE reset rather
than the PASSIVE backfill.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
48b7d20a0bJulius Marminge committed on 10/22/2026, 8:48:23 AMparent26d9595

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