ReviewIdleQueue stored (agent, session_key, kwargs, enqueued_at) with no
context, and the shared dispatcher thread called _still_enabled(item) and
item.agent._spawn_background_review_now(**item.kwargs) under its own
AMBIENT context. On a multiplexed gateway that meant the wrong profile's
background_review.enabled gate decided whether a queued review ran, and the
spawned worker inherited the ambient home / no secret scope (the
propagate_context_to_thread in _spawn_background_review_now copies a
context that no longer carries the originating profile).
Capture contextvars.copy_context() at enqueue() on _PendingReview and run
both the enabled re-check and the spawn via item.context.run(...).
Test: two profile homes (enabled / disabled) plus a disabled ambient home
under set_multiplex_active(True); only the enabled profile's item spawns,
and it observes its own home override + secret scope.
Salvaged from #108538 by @Liuzikaii (mechanism kept, test trimmed into the
existing test module).
Fixes#108537
(cherry picked from commit 4f8a64fd680e40de270a18b52c8f44a414334c56)
a940858290