[fix][test] Fix flaky ProducerCleanupTest timer cleanup

[oneby-wang]

Motivation

ProducerCleanupTest#testAllTimerTaskShouldCanceledAfterProducerClosed is flaky because the test asserted HashedWheelTimer.pendingTimeouts() on the shared test client immediately after producer close. The producer close path cancels the send timeout and producer stats timeout, but Netty's HashedWheelTimer removes cancelled timeouts asynchronously in the timer worker. Under load, pendingTimeouts() can still report the cancelled tasks for a short period.

Example failure:

2026-05-23T08:07:31,286 - INFO  - [BookieDeathWatcher-53904:BookieServer] - BookieDeathWatcher exited loop due to uncaught exception from thread BookieDeathWatcher-53904 {}
java.lang.RuntimeException: Bookie is not running any more
	at org.apache.bookkeeper.proto.BookieServer$DeathWatcher.run(BookieServer.java:289)
2026-05-23T08:07:31,286 - INFO  - [BookieDeathWatcher-53904:BookieServer] - Shutting down BookieServer {}
2026-05-23T08:07:31,286 - INFO  - [BookieDeathWatcher-53904:BookieNettyServer] - Shutting down BookieNettyServer {}

Expected :0
Actual   :2

java.lang.AssertionError: expected [0] but found [2]
	at org.testng.Assert.fail(Assert.java:111)
	at org.testng.Assert.failNotEquals(Assert.java:1590)
	at org.testng.Assert.assertEqualsImpl(Assert.java:150)
	at org.testng.Assert.assertEquals(Assert.java:132)
	at org.testng.Assert.assertEquals(Assert.java:992)
	at org.testng.Assert.assertEquals(Assert.java:956)
	at org.testng.Assert.assertEquals(Assert.java:1002)
	at org.apache.pulsar.client.api.ProducerCleanupTest.testAllTimerTaskShouldCanceledAfterProducerClosed(ProducerCleanupTest.java:40)

Modifications

• Use a dedicated PulsarClient in the test so the timer assertion only observes tasks created by this test.
• Set the producer send timeout to 15 seconds. This keeps the timeout task in the initial state long enough for producer.close() to exercise the cancellation path.
• Replace the fixed Thread.sleep(2000) and one-shot assertion with Awaitility, waiting up to 10 seconds for Netty's timer worker to clean up cancelled timeouts.

Verifying this change

• Make sure that the change passes the CI checks.

If the box was checked, please highlight the changes

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

[lhotari]

LGTM