Fold the slash-worker subprocess close into _finalize_session itself —
the single _finalized-guarded session-end chokepoint — instead of
relying on each caller (_teardown_session, _shutdown_sessions) to close
it separately. A future code path that finalizes a session directly can
no longer reintroduce the #38095 worker leak.
Idempotent: _SlashWorker.close() is poll()-guarded and _finalize_session
short-circuits on _finalized, so the existing teardown paths are
unaffected. Drops the now-redundant separate close() in
_shutdown_sessions.
Note: the active leak this issue reported was already fixed on main
(WS-orphan reaper #38591, _restart_slash_worker close, atexit shutdown).
This addresses the residual defense-in-depth gap the reporter correctly
identified in their follow-up comment.
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