Refine multipart converter registration in HttpMessageConverters

As of #33894, `HttpMessageConverters` auto-detects converters and use
custom-provided ones to configure a collection of converters for the
client or the server.
Right now the multipart converter is only configured if core converters
(JSON, XML...) are configured/detected. We do not reuse the base
converters (resource, string, byte array) for the multipart converter
as it applies different encoding defaults (ISO for the main ones, UTF-8
for multipart).

This commit refines the configuration to not only include the multipart
converter when core converters are present, but also if any other
converter was configured.

Closes gh-35203

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

@@ -160,6 +160,13 @@ class DefaultHttpMessageConvertersTests {
 			assertThat(multipartConverter.getPartConverters()).hasAtLeastOneElementOfType(CustomHttpMessageConverter.class);
 		}
 
+		@Test
+		void registerMultipartConverterWhenOtherConvertersPresent() {
+			var converters = HttpMessageConverters.forClient()
+					.stringMessageConverter(new StringHttpMessageConverter()).build();
+			assertThat(converters).hasExactlyElementsOfTypes(StringHttpMessageConverter.class, AllEncompassingFormHttpMessageConverter.class);
+		}
+
 		@Test
 		void shouldUseSpecificConverter() {
 			var jacksonConverter = new JacksonJsonHttpMessageConverter();
@@ -248,7 +255,14 @@ class DefaultHttpMessageConvertersTests {
 		}
 
 		@Test
-		void shouldUseServerSpecificConverter() {
+		void registerMultipartConverterWhenOtherConvertersPresent() {
+			var converters = HttpMessageConverters.forServer()
+					.stringMessageConverter(new StringHttpMessageConverter()).build();
+			assertThat(converters).hasExactlyElementsOfTypes(StringHttpMessageConverter.class, AllEncompassingFormHttpMessageConverter.class);
+		}
+
+		@Test
+		void shouldUseSpecificConverter() {
 			var jacksonConverter = new JacksonJsonHttpMessageConverter();
 			var converters = HttpMessageConverters.forServer().registerDefaults()
 					.jsonMessageConverter(jacksonConverter).build();

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

@@ -33,6 +33,7 @@ import org.springframework.http.client.JettyClientHttpRequestFactory;
 import org.springframework.http.client.support.BasicAuthenticationInterceptor;
 import org.springframework.http.converter.HttpMessageConverter;
 import org.springframework.http.converter.StringHttpMessageConverter;
+import org.springframework.http.converter.support.AllEncompassingFormHttpMessageConverter;
 import org.springframework.web.util.DefaultUriBuilderFactory;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -150,7 +151,7 @@ public class RestClientBuilderTests {
 
 		assertThat(fieldValue("messageConverters", defaultBuilder))
 				.asInstanceOf(InstanceOfAssertFactories.LIST)
-				.containsExactly(stringConverter);
+				.hasExactlyElementsOfTypes(StringHttpMessageConverter.class, AllEncompassingFormHttpMessageConverter.class);
 	}
 
 	@Test