The early-EOF reaper made _finish_reader return without publishing when
wait() raises, so the session stays tracked for later reconciliation.
That is right for the pipe path (_reconcile_local_exit can still reap via
session.process), but PTY sessions have no session.process: when
ptyprocess.wait raises (waitpid ECHILD after isalive() already reaped the
child) the exitstatus is known, yet poll() reported "running" forever.
Only leave the session tracked when exit_code() is still None; otherwise
record the known status and finish as before.
Review finding: PTY session whose pty.wait raises stays in _running forever (fail-open regression vs main)
0ea0c53e89