Replace pg_mblen() with bounds-checked versions.

Replace pg_mblen() with bounds-checked versions.

A corrupted string could cause code that iterates with pg_mblen() to
overrun its buffer.  Fix, by converting all callers to one of the
following:

1. Callers with a null-terminated string now use pg_mblen_cstr(), which
raises an "illegal byte sequence" error if it finds a terminator in the
middle of the sequence.

2. Callers with a length or end pointer now use either
pg_mblen_with_len() or pg_mblen_range(), for the same effect, depending
on which of the two seems more convenient at each site.

3. A small number of cases pre-validate a string, and can use
pg_mblen_unbounded().

The traditional pg_mblen() function and COPYCHAR macro still exist for
backward compatibility, but are no longer used by core code and are
hereby deprecated.  The same applies to the t_isXXX() functions.

Security: CVE-2026-2006
Backpatch-through: 14
Co-authored-by: Thomas Munro <thomas.munro@gmail.com>
Co-authored-by: Noah Misch <noah@leadboat.com>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reported-by: Paul Gerste (as part of zeroday.cloud)
Reported-by: Moritz Sanft (as part of zeroday.cloud)