[fix][test] Fix flaky test NonPersistentTopicTest.testMsgDropStat, follow up

[lhotari]

Fixes #20386

Motivation

• the previous attempt [fix][test] Fix flaky test NonPersistentTopicTest.testMsgDropStat #20387 to fix the flakiness wasn't effective

Modifications

• improve the fix and rely on the fact that entryId is -1 when the message is dropped in the broker
  code:

  pulsar/pulsar-broker/src/main/java/org/apache/pulsar/broker/service/ServerCnx.java

  Lines 1699 to 1711 in 091ee25

  	// avoid processing non-persist message if reached max concurrent-message limit
  	if (nonPersistentPendingMessages > maxNonPersistentPendingMessages) {
  	final long producerId = send.getProducerId();
  	final long sequenceId = send.getSequenceId();
  	final long highestSequenceId = send.getHighestSequenceId();
  	service.getTopicOrderedExecutor().executeOrdered(producer.getTopic().getName(), () -> {
  	commandSender.sendSendReceiptResponse(producerId, sequenceId, highestSequenceId, -1, -1);
  	});
  	producer.recordMessageDrop(send.getNumMessages());
  	return;
  	} else {
  	nonPersistentPendingMessages++;
  	}

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete