The salvaged fix's two regression tests mock adapter.handle_message, so
they only assert the pre-claimed sentinel is set/cleaned around a stub —
they never drive the real dispatch chain. Add a full-path test that
exercises _schedule_resume_pending_sessions -> _guarded_handle_message ->
adapter.handle_message -> _process_message_background -> _handle_message
and asserts the resumed session's agent runs EXACTLY ONCE: not zero (the
pre-claim must not self-bounce the resume into a queued no-op) and not
twice (the duplicate-agent bug #45456 the fix targets). Also assert no
leaked sentinel and no orphaned pending event after the drain settles.
Tighten the _guarded_handle_message docstring: on current main the real
sentinel is taken over inside _handle_message (not _process_message_background),
and note the `is _AGENT_PENDING_SENTINEL` guard only releases the slot we
ourselves placed, never one a live run owns.
kshitijk4poor
63097ee0d7