Support JAXBElement in Jaxb2XmlEncoder

This commit introduces support for JAXBElements in the Jaxb2XmlEncoder.

Closes gh-30552

spring-web/src/test/java/org/springframework/http/codec/xml/Jaxb2XmlEncoderTests.java

@@ -20,6 +20,9 @@ import java.util.Arrays;
 import java.util.List;
 import java.util.function.Consumer;
 
+import javax.xml.namespace.QName;
+
+import jakarta.xml.bind.JAXBElement;
 import jakarta.xml.bind.annotation.XmlElement;
 import jakarta.xml.bind.annotation.XmlElements;
 import jakarta.xml.bind.annotation.XmlRootElement;
@@ -76,6 +79,18 @@ public class Jaxb2XmlEncoderTests extends AbstractEncoderTests<Jaxb2XmlEncoder>
 				.verifyComplete());
 	}
 
+	@Test
+	public void encodeJaxbElement() {
+		Mono<JAXBElement<Pojo>> input = Mono.just(new JAXBElement<>(new QName("baz"), Pojo.class,
+				new Pojo("foofoo", "barbar")));
+
+		testEncode(input, Pojo.class, step -> step
+				.consumeNextWith(expectXml(
+						"<?xml version='1.0' encoding='UTF-8' standalone='yes'?>" +
+								"<baz><bar>barbar</bar><foo>foofoo</foo></baz>"))
+				.verifyComplete());
+	}
+
 	@Test
 	public void encodeError() {
 		Flux<Pojo> input = Flux.error(RuntimeException::new);