[PG19] btree_gin misses rows for a timestamptz bound in the repeated DST hour

Hi hackers,

This is Medium 4 from my list in [1].

Since fc896821c44, a btree_gin index on a timestamp column can be
used to compare with a timestamptz value. When that value is in the
first pass of the repeated hour at the end of DST, the index scan
misses rows.

create extension btree_gin;
set timezone = 'America/New_York';
create table t (ts timestamp);
insert into t values ('2026-11-01 01:10'), ('2026-11-01 01:40');
create index on t using gin (ts);
set enable_seqscan = off;
select * from t where ts >= '2026-11-01 01:30-04'::timestamptz;

master returns only 01:40. 18 returns both rows, and so does master
with a seq scan.

0001 converts the start key back to timestamptz. If the result is
after the bound, it moves the key earlier by the difference. Here the
key becomes 00:30. The scan already rechecks each entry with the real
operator, so nothing extra is returned. 0002 adds a test. The patches
apply to master and REL_19_STABLE.

A date column goes through the same kind of conversion. I could not
make it fail, also not in the Cuba time zone where midnight repeats.
A bound in the spring gap can give wrong rows too, but a plain btree
index does the same there, so I left it alone.

The commit came from [2].

[1]
https://postgr.es/m/CAGRkXqSFdjqZP_kwRz7dv6LHX-CSrkDnynNXgntRsCEnQTKxmg@mail.gmail.com
[2] https://postgr.es/m/262624.1738460652@sss.pgh.pa.us

Thanks,
Shihao