Fix SSE flushing in SseServerResponse

Prior to this commit, `SseServerResponse.send()` would flush the
output stream returned by `getBody()`. Since gh-36385,
`ServletServerHttpResponse` wraps this stream with a non-flushing
decorator to avoid performance issues with `HttpMessageConverter`
implementations that flush excessively. As a result, SSE events were
no longer flushed to the client.

This commit changes `send()` to call `outputMessage.flush()` instead
of `body.flush()`, which properly delegates to the servlet response
`flushBuffer()` and is not affected by the non-flushing wrapper.

Fixes gh-36537

spring-webmvc/src/main/java/org/springframework/web/servlet/function/SseServerResponse.java

@@ -17,7 +17,6 @@
 package org.springframework.web.servlet.function;
 
 import java.io.IOException;
-import java.io.OutputStream;
 import java.nio.charset.StandardCharsets;
 import java.time.Duration;
 import java.util.Collections;
@@ -137,9 +136,8 @@ final class SseServerResponse extends AbstractServerResponse {
 		public void send() throws IOException {
 			this.builder.append('\n');
 			try {
-				OutputStream body = this.outputMessage.getBody();
-				body.write(builderBytes());
-				body.flush();
+				this.outputMessage.getBody().write(builderBytes());
+				this.outputMessage.flush();
 			}
 			catch (IOException ex) {
 				this.sendFailed = true;

spring-webmvc/src/test/java/org/springframework/web/servlet/function/SseServerResponseTests.java

@@ -75,6 +75,41 @@ class SseServerResponseTests {
 		assertThat(this.mockResponse.getContentAsString()).isEqualTo(expected);
 	}
 
+	@Test // gh-36537
+	void sendStringFlushesResponse() throws Exception {
+		ServerResponse response = ServerResponse.sse(sse -> {
+			try {
+				sse.send("foo bar");
+			}
+			catch (IOException ex) {
+				throw new UncheckedIOException(ex);
+			}
+		});
+
+		ServerResponse.Context context = Collections::emptyList;
+
+		response.writeTo(this.mockRequest, this.mockResponse, context);
+		assertThat(this.mockResponse.isCommitted()).isTrue();
+	}
+
+	@Test // gh-36537
+	void sendObjectFlushesResponse() throws Exception {
+		Person person = new Person("John Doe", 42);
+		ServerResponse response = ServerResponse.sse(sse -> {
+			try {
+				sse.send(person);
+			}
+			catch (IOException ex) {
+				throw new UncheckedIOException(ex);
+			}
+		});
+
+		ServerResponse.Context context = () -> List.of(new JacksonJsonHttpMessageConverter());
+
+		response.writeTo(this.mockRequest, this.mockResponse, context);
+		assertThat(this.mockResponse.isCommitted()).isTrue();
+	}
+
 	@Test
 	void sendObject() throws Exception {
 		Person person = new Person("John Doe", 42);