BUG #19519: REPACK can fail due to missing chunk for toast value

The following bug has been logged on the website:

Bug reference: 19519
Logged by: Alexander Lakhin
Email address: exclusion(at)gmail(dot)com
PostgreSQL version: 19beta1
Operating system: Ubuntu 24.04
Description:

The following script:
createdb db1
createdb db2

cat << EOF | psql db1
SET default_statistics_target = 1000;
CREATE TABLE t(i int, t text);
INSERT INTO t SELECT 1, g::text FROM generate_series(1, 50000) g;
ANALYZE t;
EOF

cat << EOF | psql db2 &
BEGIN;
CREATE TABLE t(i int);
SELECT pg_sleep(3);
EOF

sleep 1

cat << EOF | psql db1
DROP TABLE t;
VACUUM pg_toast.pg_toast_2619;
REPACK;
EOF
wait

triggers:
ERROR: missing chunk number 0 for toast value 16393 in pg_toast_2619

Reproduced starting from ac58465e0.