Add tests for SSE heartbeat (ping) functionality

This commit adds test cases to verify the heartbeat (ping) feature in SseEmitter. The tests ensure that heartbeat messages are sent at the specified interval and that they stop after the emitter is completed.

See: spring-projectsgh-33355

Author: Torres-09

spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/annotation/SseEmitterTests.java

@@ -137,6 +137,45 @@ void sendEventFullWithTwoDataLinesInTheMiddle() throws Exception {
 		this.handler.assertWriteCount(1);
 	}
 
+	@Test
+	void heartbeatIsSent() throws Exception {
+		this.emitter = new SseEmitter(0L, 100L);
+		this.emitter.initialize(this.handler);
+		Thread.sleep(250);
+		int heartbeatCount = 0;
+		for (int i = 0; i < this.handler.objects.size(); i++) {
+			Object data = this.handler.objects.get(i);
+			if (data.equals(":heartbeat\n\n")) {
+				heartbeatCount++;
+			}
+		}
+		assertThat(heartbeatCount).isGreaterThanOrEqualTo(2);
+	}
+
+	@Test
+	void heartbeatStopsAfterCompletion() throws Exception {
+		this.emitter = new SseEmitter(0L, 100L);
+		this.emitter.initialize(this.handler);
+
+		Thread.sleep(150);
+		this.emitter.complete();
+
+		int heartbeatCountBeforeCompletion = 0;
+		for (Object data : this.handler.objects) {
+			if (data.equals(":heartbeat\n\n")) {
+				heartbeatCountBeforeCompletion++;
+			}
+		}
+		Thread.sleep(150);
+		int totalHeartbeatCount = 0;
+		for (Object data : this.handler.objects) {
+			if (data.equals(":heartbeat\n\n")) {
+				totalHeartbeatCount++;
+			}
+		}
+		assertThat(totalHeartbeatCount).isEqualTo(heartbeatCountBeforeCompletion);
+	}
+
 
 	private static class TestHandler implements ResponseBodyEmitter.Handler {