Add tests for cross-session temp table access

Add tests for cross-session temp table access

Add a TAP test in src/test/modules/test_misc that documents what
happens when one session attempts to read or modify another session's
temporary table.  This commit only adds tests; it does not change
backend behavior, so the assertions reflect current behavior:

- SELECT, UPDATE, DELETE, MERGE, COPY on a table without an index
  silently succeed with no error and zero rows / zero affected rows.
  These commands run through the read-stream path, which currently
  bypasses the RELATION_IS_OTHER_TEMP() check.  This is the
  underlying bug to be fixed in a follow-up.
- INSERT errors with "cannot access temporary tables of other
  sessions" because hio.c calls ReadBufferExtended() to find a page
  with free space and is caught by the existing check there.
- Index scan errors via the same existing check, reached through
  nbtree -> ReadBuffer -> ReadBufferExtended.
- TRUNCATE / ALTER TABLE / ALTER INDEX / CLUSTER fail with their
  command-specific error messages.
- VACUUM is silently skipped to avoid noise during database-wide
  VACUUM (vacuum_rel() returns without warning).
- DROP TABLE is intentionally allowed: DROP does not touch the
  table's contents, and autovacuum relies on this to clean up
  temp relations orphaned by a crashed backend.
- ALTER FUNCTION / DROP FUNCTION on an owner-created function over
  its own temp row type work as catalog operations -- they don't
  read the underlying data.
- CREATE FUNCTION from a separate session, using another session's
  temp row type as an argument, is allowed but emits a NOTICE: the
  function is moved into the creator's pg_temp namespace with an
  auto-dependency on the borrowed type, so it disappears together
  with the session that created it.
- A bare DROP TABLE on a temp table that has a cross-session
  dependent function fails with a catalog-level dependency error.
- LOCK TABLE in ACCESS SHARE mode on another session's temp table
  succeeds and properly blocks the owner's session-exit cleanup
  (which acquires AccessExclusiveLock via findDependentObjects).
  This exercises the same LockRelationOid path used by autovacuum
  when cleaning up orphaned temp relations.
- When the owner session ends, the normal session-exit cleanup
  cascades through DEPENDENCY_NORMAL and removes both the temp
  objects and any cross-session functions that depended on them.

Also, document the contract for RELATION_IS_OTHER_TEMP() so that
future buffer-access entry points enforce the same rule.

Backpatch this through PostgreSQL 17, where b7b0f3f27241 introduces a code
path bypassing this check.

Author: Jim Jones <jim.jones@uni-muenster.de>
Author: Daniil Davydov <3danissimo@gmail.com>
Co-authored-by: Alexander Korotkov <aekorotkov@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Soumya S Murali <soumyamurali.work@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAJDiXghdFcZ8%3Dnh4G69te7iRr3Q0uFyXxb3ZdG09_GTNZXwH0g%40mail.gmail.com
Backpatch-through: 17