perf: Schedule session end on shared executor (JAVA-653) (#5819)

* perf: Schedule session end on shared executor (JAVA-653)

LifecycleWatcher created a java.util.Timer whose thread was spawned on
the first background transition and lived for the rest of the process.
Schedule the session-end task on the shared timer executor instead,
whose single worker thread is reused and self-terminates when idle. If
scheduling fails (executor already shut down), the session is ended
right away instead of leaking.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* changelog

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Co-authored-by: Claude Fable 5 <noreply@anthropic.com>