Prior to this PR, we had:
71cae06e66/codex-rs/login/src/server.rs (L141-L142)
which means that we could be blocked waiting for a new request in
`server_for_thread.recv()` and not notice that the state of
`shutdown_flag` had changed.
With this PR, we use `shutdown_flag: Notify` so that we can
`tokio::select!` on `shutdown_notify.notified()` and `rx.recv()` (which
is the "async stream" of requests read from `server_for_thread.recv()`)
and handle whichever one happens first.
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* #2393
* #2389