Support StreamingHttpOutputMessage in RestClient

This commit allows RestClient to handle StreamingHttpOutputMessage
properly by checking the type of the request and invoking setBody()
when appropriate. This improves interoperability with components that
expect streamed output.

A new integration test has been added to verify the functionality.

See gh-35102

Signed-off-by: Daniil Razorenov <daniltmb@gmail.com>

spring-web/src/test/java/org/springframework/web/client/RestClientIntegrationTests.java

@@ -16,6 +16,7 @@
 
 package org.springframework.web.client;
 
+import java.io.ByteArrayInputStream;
 import java.io.IOException;
 import java.lang.annotation.ElementType;
 import java.lang.annotation.Retention;
@@ -529,6 +530,27 @@ class RestClientIntegrationTests {
 		});
 	}
 
+	@ParameterizedRestClientTest
+	void postStreamingMessageBody(ClientHttpRequestFactory requestFactory) {
+		startServer(requestFactory);
+
+		prepareResponse(response -> response.setResponseCode(200));
+
+		ResponseEntity<Void> result = this.restClient.post()
+				.uri("/streaming/body")
+				.body(new ByteArrayInputStream("test-data".getBytes(UTF_8))::transferTo)
+				.retrieve()
+				.toBodilessEntity();
+
+		assertThat(result.getStatusCode()).isEqualTo(HttpStatus.OK);
+
+		expectRequestCount(1);
+		expectRequest(request -> {
+			assertThat(request.getPath()).isEqualTo("/streaming/body");
+			assertThat(request.getBody().readUtf8()).isEqualTo("test-data");
+		});
+	}
+
 	@ParameterizedRestClientTest // gh-31361
 	public void postForm(ClientHttpRequestFactory requestFactory) {
 		startServer(requestFactory);