exec-server: wait for close after observed exit

[jif-oai]

Why

Windows CI can flake in server::handler::tests::output_and_exit_are_retained_after_notification_receiver_closes after a process has exited but before both output streams have closed. exec/read returned immediately whenever exited was true, so callers that had already observed the exit event could spin instead of long-polling for the later closed state.

What Changed

• Keep returning immediately when a terminal exit event is newly observable.
• Allow later reads, after the caller has advanced past that event, to wait for closed or new output until wait_ms expires.

Verification

• CI pending.