quic-radix: add thread-assisted idle keepalive test

quic-radix: add thread-assisted idle keepalive test

Add a radix vignette covering thread-assisted mode: with the client's
per-op ticking disabled, only its assist thread can act on the
connection, so skipping the virtual time past the idle timeout keeps the
server up only if the assist thread keeps sending keepalives. This
migrates the thread-assisted idle coverage away from the
QUIC_TSERVER-based test.

The client is created with OSSL_QUIC_client_thread_method() so the
method itself stays covered as well. The client and listener are linked
with an in-memory datagram BIO pair instead of real UDP sockets, so
keepalive delivery cannot race the virtual clock in the OS UDP path.

New ops: hf_set_tick_active (enable/disable per-op ticking),
hf_skip_time_wait (skip virtual time, wake the assist thread and wait
for it to catch up), hf_expect_connected, and hf_link_dgram_pair.

Assisted-by: Claude:claude-opus-4-8
Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Andrew Dinh <andrewd@openssl.org>
MergeDate: Mon Aug  3 02:12:44 2026
(Merged from https://github.com/openssl/openssl/pull/31918)