[serve] Deflake `test_replica_startup_status_transitions ` by `await`ing signal actor in constructor (#24044)
[serve] Deflake `test_replica_startup_status_transitions ` by `await`ing signal actor in constructor (#24044)

`test_cluster: test_replica_startup_status_transitions` is periodically flaky with the replica hanging in `PENDING_ALLOCATION`. This could be because there is no ordering guarantee on async actor calls, so the `reconfigure` method might execute first and block the asyncio loop (due to `ray.get`), not allowing the `is_allocated` call to run.
ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray