GH-96827: Don't touch closed loops from executor threads (GH-96837)

* When chaining futures, skip callback if loop closed.
* When shutting down an executor, don't wake a closed loop.
(cherry picked from commit e9d63760fea8748638f6e495b5b07bd1805c9591)

Co-authored-by: Guido van Rossum <guido@python.org>