Clean up metadata handling in tls_crypt_v2_extract_client_key

Clean up metadata handling in tls_crypt_v2_extract_client_key

This makes the metadata a local variable instead of a member of the
wrap_context struct. Also always ensure that this buffer is freed to
avoid any leak of the metadata buffer.

This touches the check methods. Ensure that they still work as
intended by adding unit tests for both script and age checks.

CVE: 2026-12932
Github: OpenVPN/openvpn-private-issues#133
Github: OpenVPN/openvpn-private-issues#136

Reported-By: Valton Tahiri <valton.taa@gmail.com>
Acked-by: MaxF <max@max-fillinger.net>
Change-Id: I9b8b8afcc5d4d5914b2008c0efccb309f0d07d4b