SseEmitter support for multiline String

Closes gh-30965

Author: rstoyanchev

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/SseEmitter.java

@@ -237,6 +237,9 @@ public SseEventBuilder data(Object object) {
 		public SseEventBuilder data(Object object, @Nullable MediaType mediaType) {
 			append("data:");
 			saveAppendedText();
+			if (object instanceof String text) {
+				object = StringUtils.replace(text, "\n", "\ndata:");
+			}
 			this.dataToSend.add(new DataWithMediaType(object, mediaType));
 			append('\n');
 			return this;

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

@@ -104,6 +104,16 @@ public void sendEventWithTwoDataLines() throws Exception {
 		this.handler.assertWriteCount(1);
 	}
 
+	@Test
+	public void sendEventWithMultiline() throws Exception {
+		this.emitter.send(event().data("foo\nbar\nbaz"));
+		this.handler.assertSentObjectCount(3);
+		this.handler.assertObject(0, "data:", TEXT_PLAIN_UTF8);
+		this.handler.assertObject(1, "foo\ndata:bar\ndata:baz");
+		this.handler.assertObject(2, "\n\n", TEXT_PLAIN_UTF8);
+		this.handler.assertWriteCount(1);
+	}
+
 	@Test
 	public void sendEventFull() throws Exception {
 		this.emitter.send(event().comment("blah").name("test").reconnectTime(5000L).id("1").data("foo"));