Make SourceHttpMessageConverter optional

As a follow-up to gh-29277, and since the JAXB support is now
triggered by the classpath presence of a JAXB implementation,
it makes sense to make SourceHttpMessageConverter, previously
configured unconditionally, optional.

That makes a big difference on native (1M of RSS reduction
with current typical Spring Boot 3 arrangement, 3.4M when
other usages of XML are not reachable).

It also brings more consistency between Spring MVC
and Spring WebFlux, and means that XML support for
Spring web applications now needs to be enabled explicitly.

As a consequence, Spring web applications using
javax.xml.transform.Source now needs to configure
SourceHttpMessageConverter explicitly in RestTemplate or
Spring MVC.

Closes gh-29535

spring-web/src/test/java/org/springframework/http/converter/FormHttpMessageConverterTests.java

@@ -44,11 +44,13 @@ import org.springframework.http.MediaType;
 import org.springframework.http.MockHttpInputMessage;
 import org.springframework.http.MockHttpOutputMessage;
 import org.springframework.http.converter.support.AllEncompassingFormHttpMessageConverter;
+import org.springframework.http.converter.xml.SourceHttpMessageConverter;
 import org.springframework.util.LinkedMultiValueMap;
 import org.springframework.util.MultiValueMap;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.springframework.http.MediaType.APPLICATION_FORM_URLENCODED;
+import static org.springframework.http.MediaType.APPLICATION_JSON;
 import static org.springframework.http.MediaType.MULTIPART_FORM_DATA;
 import static org.springframework.http.MediaType.MULTIPART_MIXED;
 import static org.springframework.http.MediaType.TEXT_XML;
@@ -60,6 +62,7 @@ import static org.springframework.http.MediaType.TEXT_XML;
  * @author Arjen Poutsma
  * @author Rossen Stoyanchev
  * @author Sam Brannen
+ * @author Sebastien Deleuze
  */
 public class FormHttpMessageConverterTests {
 
@@ -155,6 +158,90 @@ public class FormHttpMessageConverterTests {
 
 	@Test
 	public void writeMultipart() throws Exception {
+
+		MultiValueMap<String, Object> parts = new LinkedMultiValueMap<>();
+		parts.add("name 1", "value 1");
+		parts.add("name 2", "value 2+1");
+		parts.add("name 2", "value 2+2");
+		parts.add("name 3", null);
+
+		Resource logo = new ClassPathResource("/org/springframework/http/converter/logo.jpg");
+		parts.add("logo", logo);
+
+		// SPR-12108
+		Resource utf8 = new ClassPathResource("/org/springframework/http/converter/logo.jpg") {
+			@Override
+			public String getFilename() {
+				return "Hall\u00F6le.jpg";
+			}
+		};
+		parts.add("utf8", utf8);
+
+		MyBean myBean = new MyBean();
+		myBean.setString("foo");
+		HttpHeaders entityHeaders = new HttpHeaders();
+		entityHeaders.setContentType(APPLICATION_JSON);
+		HttpEntity<MyBean> entity = new HttpEntity<>(myBean, entityHeaders);
+		parts.add("json", entity);
+
+		Map<String, String> parameters = new LinkedHashMap<>(2);
+		parameters.put("charset", StandardCharsets.UTF_8.name());
+		parameters.put("foo", "bar");
+
+		MockHttpOutputMessage outputMessage = new MockHttpOutputMessage();
+		this.converter.write(parts, new MediaType("multipart", "form-data", parameters), outputMessage);
+
+		final MediaType contentType = outputMessage.getHeaders().getContentType();
+		assertThat(contentType.getParameters()).containsKeys("charset", "boundary", "foo"); // gh-21568, gh-25839
+
+		// see if Commons FileUpload can read what we wrote
+		FileUpload fileUpload = new FileUpload();
+		fileUpload.setFileItemFactory(new DiskFileItemFactory());
+		RequestContext requestContext = new MockHttpOutputMessageRequestContext(outputMessage);
+		List<FileItem> items = fileUpload.parseRequest(requestContext);
+		assertThat(items.size()).isEqualTo(6);
+		FileItem item = items.get(0);
+		assertThat(item.isFormField()).isTrue();
+		assertThat(item.getFieldName()).isEqualTo("name 1");
+		assertThat(item.getString()).isEqualTo("value 1");
+
+		item = items.get(1);
+		assertThat(item.isFormField()).isTrue();
+		assertThat(item.getFieldName()).isEqualTo("name 2");
+		assertThat(item.getString()).isEqualTo("value 2+1");
+
+		item = items.get(2);
+		assertThat(item.isFormField()).isTrue();
+		assertThat(item.getFieldName()).isEqualTo("name 2");
+		assertThat(item.getString()).isEqualTo("value 2+2");
+
+		item = items.get(3);
+		assertThat(item.isFormField()).isFalse();
+		assertThat(item.getFieldName()).isEqualTo("logo");
+		assertThat(item.getName()).isEqualTo("logo.jpg");
+		assertThat(item.getContentType()).isEqualTo("image/jpeg");
+		assertThat(item.getSize()).isEqualTo(logo.getFile().length());
+
+		item = items.get(4);
+		assertThat(item.isFormField()).isFalse();
+		assertThat(item.getFieldName()).isEqualTo("utf8");
+		assertThat(item.getName()).isEqualTo("Hall\u00F6le.jpg");
+		assertThat(item.getContentType()).isEqualTo("image/jpeg");
+		assertThat(item.getSize()).isEqualTo(logo.getFile().length());
+
+		item = items.get(5);
+		assertThat(item.getFieldName()).isEqualTo("json");
+		assertThat(item.getContentType()).isEqualTo("application/json");
+	}
+
+	@Test
+	public void writeMultipartWithSourceHttpMessageConverter() throws Exception {
+
+		converter.setPartConverters(List.of(
+				new StringHttpMessageConverter(),
+				new ResourceHttpMessageConverter(),
+				new SourceHttpMessageConverter<>()));
+
 		MultiValueMap<String, Object> parts = new LinkedMultiValueMap<>();
 		parts.add("name 1", "value 1");
 		parts.add("name 2", "value 2+1");