Fix WorkerFactory command-worker cleanup race

[eamsden]

What changed?

• Added a deterministic regression test for WorkerFactory.awaitTermination races with command-worker cleanup #2951. The test completes command-worker shutdown after WorkerFactory.awaitTermination has passed its null check, reproducing the cleanup race.
• Removed unnecessary workerCommandWorker = null that caused the race.

Why?

The asynchronous shutdown continuation can clear workerCommandWorker before the termination callback dereferences it. Closes #2951

Breaking changes

None.

Server PR

N/A.

Testing

./gradlew :temporal-sdk:test --offline --tests "io.temporal.worker.WorkerShutdownTest.awaitTerminationDoesNotRaceWithWorkerCommandWorkerCleanup"

The test fails as expected on current main with the reported NullPointerException. It passes when the asynchronous workerCommandWorker = null cleanup is removed.