sql: pin read sequence number while draining a WITH HOLD cursor for persistence

[sijie-Z]

persistCursor drains the cursor via bare cursor.Next() calls during the
COMMIT phase for WITH HOLD cursors, but never brackets the iteration
with SetReadSeqNum / restore. The fetchMoveNodeBase path already does
this to preserve Postgres-style cursor sensitivity; persistCursor
omits it, so the persisted rows can be stamped with whatever the
txn's read seqnum has advanced to since DECLARE.

Apply the same pin/restore in persistCursor. Add a logictest regression.

Fixes #172772

Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com

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