pgsql: 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(at)jeltef(dot)nl>
Author: Jelte Fennema-Nio <postgres(at)jeltef(dot)nl>
Reviewed-by: Nathan Bossart <nathandbossart(at)gmail(dot)com>
Reviewed-by: Tomas Vondra <tomas(at)vondra(dot)me>
Security: CVE-2026-6472
Backpatch-through: 14

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/d92852d624f3d2df4b1140349350fa2c1fdc6571
Author: Nathan Bossart <nathan(at)postgresql(dot)org>

Modified Files
--------------
src/backend/commands/typecmds.c | 7 +++++++
src/test/regress/expected/multirangetypes.out | 16 ++++++++++++++++
src/test/regress/sql/multirangetypes.sql | 16 ++++++++++++++++
3 files changed, 39 insertions(+)