pgsql: BRIN autosummarization may need a snapshot

BRIN autosummarization may need a snapshot

It's possible to define BRIN indexes on functions that require a
snapshot to run, but the autosummarization feature introduced by commit
7526e10224f0 fails to provide one.  This causes autovacuum to leave a
BRIN placeholder tuple behind after a failed work-item execution, making
such indexes less efficient.  Repair by obtaining a snapshot prior to
running the task, and add a test to verify this behavior.

Author: Álvaro Herrera <alvherre@kurilemu.de>
Reported-by: Giovanni Fabris <giovanni.fabris@icon.it>
Reported-by: Arthur Nascimento <tureba@gmail.com>
Backpatch-through: 13
Discussion: https://postgr.es/m/202511031106.h4fwyuyui6fz@alvherre.pgsql

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6ef33c8051ddfcfff32e5c788e8e7e2f5b10669b

Modified Files
--------------
src/backend/postmaster/autovacuum.c     |  2 ++
src/test/modules/brin/t/01_workitems.pl | 37 ++++++++++++++++++++++++++++++---
2 files changed, 36 insertions(+), 3 deletions(-)