fix(agtype): coerce item to agtype in IN operator to support edge/vertex membership

[waterWang]

Fixes #2542

Problem

r0 IN relationships(p0) fails with unknown type of agtype container 1610612736:

MATCH ()-[r0]->()
MATCH p0 = (:L)-[:R]->()
RETURN r0 IN relationships(p0) AS member

The same applies to vertex membership: a IN nodes(p0).

Root cause

agtype_in_operator reads the item argument (an edge or vertex variable) with AG_GET_ARG_AGTYPE_P(1), which assumes the argument is already a well-formed agtype varlena. A matched edge/vertex variable is not passed as a self-contained agtype datum here, so reading its root container header yields garbage (e.g. 0x60000000 — both array and object flags set), and agtype_iterator_init raises unknown type of agtype container.

Fix

Use get_one_agtype_from_variadic_args(fcinfo, 1, 1) for the item argument in both the VLE-path branch and the plain-array branch of agtype_in_operator. It performs proper type categorization + datum_to_agtype coercion when the argument is not already an agtype, so an edge/vertex item is wrapped into a valid agtype raw-scalar array before iterator initialization. NULL handling is preserved (returns NULL for both SQL NULL and agtype null).

Tests

Added regression tests in regress/sql/expr.sql:

• r0 IN relationships(p0) — edge membership, previously crashed with unknown type of agtype container
• a IN nodes(p0) — vertex membership, same class of bug

All existing tests pass (42/43; the age_load failure is a pre-existing environment/path issue unrelated to this change).