Ensure shutdown hook is registered when a handler is added

Closes gh-34627

Author: wilkinsona

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/SpringApplicationShutdownHook.java

@@ -170,6 +170,7 @@ private class Handlers implements SpringApplicationShutdownHandlers, Runnable {
 		@Override
 		public void add(Runnable action) {
 			Assert.notNull(action, "Action must not be null");
+			addRuntimeShutdownHookIfNecessary();
 			synchronized (SpringApplicationShutdownHook.class) {
 				assertNotInProgress();
 				this.actions.add(action);

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/SpringApplicationShutdownHookTests.java

@@ -57,6 +57,15 @@ void shutdownHookIsNotAddedUntilContextIsRegistered() {
 		assertThat(shutdownHook.isRuntimeShutdownHookAdded()).isTrue();
 	}
 
+	@Test
+	void shutdownHookIsNotAddedUntilHandlerIsRegistered() {
+		TestSpringApplicationShutdownHook shutdownHook = new TestSpringApplicationShutdownHook();
+		assertThat(shutdownHook.isRuntimeShutdownHookAdded()).isFalse();
+		shutdownHook.getHandlers().add(() -> {
+		});
+		assertThat(shutdownHook.isRuntimeShutdownHookAdded()).isTrue();
+	}
+
 	@Test
 	void runClosesContextsBeforeRunningHandlerActions() {
 		TestSpringApplicationShutdownHook shutdownHook = new TestSpringApplicationShutdownHook();