pgsql: Check for USAGE privilege on the composite type in ALTER TABLE O

Check for USAGE privilege on the composite type in ALTER TABLE OF.

This omission allowed roles without USAGE on a type to create
tables that depend on it, which could prevent the owner from
changing the type later.

Reported-by: Nathan Bossart <nathandbossart(at)gmail(dot)com>
Author: Nathan Bossart <nathandbossart(at)gmail(dot)com>
Reviewed-by: Robert Haas <robertmhaas(at)gmail(dot)com>
Security: CVE-2026-6470
Backpatch-through: 14

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/c062734cdb8a7a966f4e39bda47279b9178f5da0
Author: Nathan Bossart <nathan(at)postgresql(dot)org>

Modified Files
--------------
src/backend/commands/tablecmds.c | 5 +++++
src/test/regress/expected/privileges.out | 6 ++++++
src/test/regress/sql/privileges.sql | 5 +++++
3 files changed, 16 insertions(+)