BUG #19898: int4_avg_accum_inv wraps below INT64_MIN instead of raising an error

The following bug has been logged on the website:

Bug reference: 19898
Logged by: Noah
Email address: imchifan8(at)gmail(dot)com
PostgreSQL version: 18.6
Operating system: Linux/amd64
Description:

pg_catalog.int4_avg_accum_inv silently wraps an inverse-transition sum below
INT64_MIN. Given a reachable state with count 4294967298 and sum INT64_MIN,
removing the input value 1 returns a sum of INT64_MAX instead of reporting
numeric-value-out-of-range.

The state is mathematically reachable by adding 1, then 2^32 copies of
INT32_MIN, then -1. Removing the leading 1 makes the exact next sum
INT64_MIN - 1, which cannot be represented by bigint and should raise an
out-of-range error.

How to repeat:

\set ON_ERROR_STOP on
-- This state is mathematically reachable: +1, followed by 2^32 copies of
-- INT32_MIN, followed by -1, yields count 4294967298 and sum INT64_MIN.
-- Removing the leading +1 makes the exact sum INT64_MIN - 1.
SELECT pg_catalog.int4_avg_accum_inv(
ARRAY[4294967298::bigint, '-9223372036854775808'::bigint],
1::integer) AS inverse_state;

The output was:

inverse_state
----------------------------------
{4294967297,9223372036854775807}
(1 row)

The returned count is reduced by one as expected, but the sum wraps from the
unrepresentable INT64_MIN - 1 result to INT64_MAX.

The issue was reproduced on PostgreSQL 18.6 on Linux/amd64. This occurs when
an int4 moving aggregate removes a value from a reachable state whose exact
next sum is below INT64_MIN.