Check CREATE privilege on multirange type schema in CREATE TYPE.

Check CREATE privilege on multirange type schema in CREATE TYPE.

This omission allowed roles to create multirange types in any
schema, potentially leading to privilege escalations.  Note that
when a multirange type name is not specified in CREATE TYPE, it is
automatically placed in the range type's schema, which is checked
at the beginning of DefineRange().

Reported-by: Jelte Fennema-Nio <postgres@jeltef.nl>
Author: Jelte Fennema-Nio <postgres@jeltef.nl>
Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Reviewed-by: Tomas Vondra <tomas@vondra.me>
Security: CVE-2026-6472
Backpatch-through: 14