pgsql: Validate publisher for retain_dead_tuples in the apply worker.

Validate publisher for retain_dead_tuples in the apply worker.

Enabling retain_dead_tuples requires the publisher to run PostgreSQL 19 or
later and to not be in recovery. Previously this was checked only at DDL
time. That forced ALTER SUBSCRIPTION ... ENABLE to connect to the
publisher, so pg_upgrade (which re-enables subscriptions during restore)
failed if the publisher was unreachable. It was also not authoritative,
since the publisher's version or recovery status can change afterwards,
for example after a failover.

Perform the check authoritatively in the apply worker when it connects,
and stop doing it when enabling a subscription. ENABLE is the only command
issued during restore that triggered it, so this also fixes the pg_upgrade
failure. The DDL-time check is kept as a convenience for the other paths,
none of which are issued during restore.

Reported-by: Noah Misch <noah(at)leadboat(dot)com>
Analyzed-by: Jeff Davis <pgsql(at)j-davis(dot)com>
Author: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Reviewed-by: Jeff Davis <pgsql(at)j-davis(dot)com>
Reviewed-by: Hayato Kuroda <kuroda(dot)hayato(at)fujitsu(dot)com>
Backpatch-through: 19, where it was introduced
Discussion: https://postgr.es/m/20260710195902.4f.noahmisch@microsoft.com

Branch
------
REL_19_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/98bcb0ab8aa3c783adc27e677ab861a4a8baa411

Modified Files
--------------
src/backend/commands/subscriptioncmds.c | 22 +++++++++-------------
src/backend/replication/logical/worker.c | 15 +++++++++++++++
src/include/commands/subscriptioncmds.h | 4 ++++
3 files changed, 28 insertions(+), 13 deletions(-)