Allow to disable defaults in HttpMessageConverters

The builder for `HttpMessageConverters` allows for auto-detection of
message converters on the classpath and their default registration when
`registerDefaults()` is called. Once called, there is no way to undo
this.

This commit adds a new `disableDefaults()` method to disable the default
registration and take full control over the list of message converters.

Closes gh-36303

spring-web/src/main/java/org/springframework/http/converter/DefaultHttpMessageConverters.java

@@ -389,6 +389,12 @@ class DefaultHttpMessageConverters implements HttpMessageConverters {
 			return this;
 		}
 
+		@Override
+		public ClientBuilder disableDefaults() {
+			this.registerDefaults = false;
+			return this;
+		}
+
 		@Override
 		public ClientBuilder withStringConverter(HttpMessageConverter<?> stringConverter) {
 			setStringConverter(stringConverter);
@@ -495,6 +501,12 @@ class DefaultHttpMessageConverters implements HttpMessageConverters {
 			return this;
 		}
 
+		@Override
+		public ServerBuilder disableDefaults() {
+			this.registerDefaults = false;
+			return this;
+		}
+
 		@Override
 		public ServerBuilder withStringConverter(HttpMessageConverter<?> stringConverter) {
 			setStringConverter(stringConverter);

spring-web/src/main/java/org/springframework/http/converter/HttpMessageConverters.java

@@ -97,6 +97,15 @@ public interface HttpMessageConverters extends Iterable<HttpMessageConverter<?>>
 		 */
 		T registerDefaults();
 
+		/**
+		 * Prevent the registration of default converters using classpath detection.
+		 * Manual registrations like {@link #withJsonConverter(HttpMessageConverter)} are
+		 * still allowed. If both {@link #registerDefaults()} and this method are called,
+		 * the latest call wins.
+		 * @since 7.0.4
+		 */
+		T disableDefaults();
+
 		/**
 		 * Override the default String {@code HttpMessageConverter}
 		 * with any converter supporting String conversion.

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

@@ -125,6 +125,12 @@ class DefaultHttpMessageConvertersTests {
 					RssChannelHttpMessageConverter.class);
 		}
 
+		@Test
+		void disableDefaults() {
+			var converters = HttpMessageConverters.forClient().registerDefaults().disableDefaults().build();
+			assertThat(converters).isEmpty();
+		}
+
 		@Test
 		void multipartConverterContainsOtherConverters() {
 			var converters = HttpMessageConverters.forClient().registerDefaults().build();
@@ -242,6 +248,12 @@ class DefaultHttpMessageConvertersTests {
 					RssChannelHttpMessageConverter.class);
 		}
 
+		@Test
+		void disableDefaults() {
+			var converters = HttpMessageConverters.forServer().registerDefaults().disableDefaults().build();
+			assertThat(converters).isEmpty();
+		}
+
 		@Test
 		void multipartConverterContainsOtherConverters() {
 			var converters = HttpMessageConverters.forServer().registerDefaults().build();