BUG #19354: JOHAB rejects valid byte sequences

The following bug has been logged on the website:

Bug reference:      19354
Logged by:          Jeroen Vermeulen
Email address:      jtvjtv@gmail.com
PostgreSQL version: 18.1
Operating system:   Debian unstable x86-64, macOS, Windows, etc.
Description:

Calling libpq, connecting to a UTF8 database and successfully setting client
encoding to JOHAB, this statement:

    PQexec(connection, "SELECT '\x8a\x5c'");

Returned an empty result with this error message:

    ERROR:  invalid byte sequence for encoding "JOHAB": 0x8a 0x5c

AFAICT, 0x8a 0x5c is a valid JOHAB sequence making up Hangul character "굎".
Easily verified in Python:

    print(b'\x8a\x5c'.decode('johab'))

It's the same story for some other valid sequences I tried, including this
character's "neighbours" 0x8a 0x5b and 0x8a 0x5d.

My test code did work with similar two-byte characters in BIG5, GB18030,
UTF-8, SJIS, and UHC.  It just breaks with these JOHAB characters on all of
these x86-64 docker images: "archlinux", "debian", "debian:unstable",
"fedora", and "ubuntu".  And I got the same results on macOS+homebrew,
Windows+MinGW with pacman-installed postgres, and a native Windows VM with
whatever-postgres-they-preinstall.