Refine test and update whats-new

This commit applies the final polishing suggestions from the review.

- The test case is refactored to be more robust and efficient. It now
  verifies the warning log message via a spy instead of relying on
  unstable timing assertions. The shutdown timeout is also reduced to
  speed up the build.

- The `whats-new.adoc` document is updated with the title and
  description suggested in the review.

Signed-off-by: Jeongjun Min <[email protected]>

Author: m3k0813

spring-rabbit/src/test/java/org/springframework/amqp/rabbit/listener/SimpleMessageListenerContainerTests.java

@@ -736,26 +736,28 @@ void testShutdownWithPendingReplies() {
 
 		template.setReplyAddress(container.getQueueNames()[0]);
 
-		long shutdownTimeout = 2000L;
+		long shutdownTimeout = 500L;
 		container.setShutdownTimeout(shutdownTimeout);
 
-		ActiveObjectCounter<Object> replyCounter =
-				(ActiveObjectCounter<Object>) ReflectionTestUtils.getField(template, "pendingRepliesCounter");
+		ActiveObjectCounter<Object> replyCounter = template.getPendingReplyCounter();
 		assertThat(replyCounter).isNotNull();
 
 		Object pending = new Object();
 		replyCounter.add(pending);
 		assertThat(replyCounter.getCount()).isEqualTo(1);
 
+		Log logger = spy(TestUtils.getPropertyValue(container, "logger", Log.class));
+		new DirectFieldAccessor(container).setPropertyValue("logger", logger);
+
 		container.start();
 
-		long startTime = System.currentTimeMillis();
 		container.stop();
-		long stopDuration = System.currentTimeMillis() - startTime;
 
-		replyCounter.release(pending);
+		await().atMost(Duration.ofMillis(500)).untilAsserted(() ->
+				verify(logger).warn("Shutdown timeout expired, but 1 pending replies still remain.")
+		);
 
-		assertThat(stopDuration).isGreaterThanOrEqualTo(shutdownTimeout - 500);
+		replyCounter.release(pending);
 	}
 
 	private Answer<Object> messageToConsumer(final Channel mockChannel, final SimpleMessageListenerContainer container,

src/reference/antora/modules/ROOT/pages/whats-new.adoc

@@ -38,7 +38,7 @@ Respective new classes have been introduced to support Jackson 3.
 
 See xref:amqp/message-converters.adoc[] for more information.
 
-[[x40-enhancements]]
-=== Enhancements
+[[x40-smlc-changes]]
+=== MessageListenerContainer Changes
 
-Defer `SimpleMessageListenerContainer` shutdown for pending `RabbitTemplate` replies.
+The `SimpleMessageListenerContainer` now awaits at most `shutdownTimeout` for pending replies from the provided `RabbitTemplate` listener on its shutdown.