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 <[email protected]>
Reported-by: Giovanni Fabris <[email protected]>
Reported-by: Arthur Nascimento <[email protected]>
Backpatch-through: 13
Discussion: https://postgr.es/m/202511031106[email protected]