_find_free_port() closed its probe socket before HTTPServer re-bound
the port minutes later, leaving a window where another process could
steal it (#22161 by @amathxbt). _reserve_callback_port() now keeps the
selected socket bound (bounded FIFO pool) until _wait_for_callback
adopts it via bind_and_activate=False. Also sets allow_reuse_address
BEFORE binding — the cherry-picked #44872 set it after the constructor
had already bound, where it is a no-op.
Also updates the three #57836 non-interactive-guard tests to the
closure-factory API from #44872.
95a0f9c836