Improve converter config in RestTestClient.Builder

Compose consumers and apply them together from the build method
to allow multiple parties to supply consumers that collaborate
on the same HttpMessageConverters.Builder.

See gh-35578

spring-test/src/main/java/org/springframework/test/web/servlet/client/DefaultRestTestClientBuilder.java

@@ -19,6 +19,8 @@ package org.springframework.test.web.servlet.client;
 import java.util.List;
 import java.util.function.Consumer;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.client.ClientHttpRequestFactory;
 import org.springframework.http.client.ClientHttpRequestInterceptor;
@@ -53,6 +55,8 @@ class DefaultRestTestClientBuilder<B extends RestTestClient.Builder<B>> implemen
 
 	private final RestClient.Builder restClientBuilder;
 
+	private @Nullable Consumer<HttpMessageConverters.ClientBuilder> convertersConfigurer;
+
 	private Consumer<EntityExchangeResult<?>> entityResultConsumer = result -> {};
 
 
@@ -66,6 +70,7 @@ class DefaultRestTestClientBuilder<B extends RestTestClient.Builder<B>> implemen
 
 	DefaultRestTestClientBuilder(DefaultRestTestClientBuilder<B> other) {
 		this.restClientBuilder = other.restClientBuilder.clone();
+		this.convertersConfigurer = other.convertersConfigurer;
 		this.entityResultConsumer = other.entityResultConsumer;
 	}
 
@@ -132,7 +137,8 @@ class DefaultRestTestClientBuilder<B extends RestTestClient.Builder<B>> implemen
 
 	@Override
 	public <T extends B> T configureMessageConverters(Consumer<HttpMessageConverters.ClientBuilder> configurer) {
-		this.restClientBuilder.configureMessageConverters(configurer);
+		this.convertersConfigurer = (this.convertersConfigurer != null ?
+				this.convertersConfigurer.andThen(configurer) : configurer);
 		return self();
 	}
 
@@ -154,6 +160,11 @@ class DefaultRestTestClientBuilder<B extends RestTestClient.Builder<B>> implemen
 
 	@Override
 	public RestTestClient build() {
+
+		if (this.convertersConfigurer != null) {
+			this.restClientBuilder.configureMessageConverters(this.convertersConfigurer);
+		}
+
 		return new DefaultRestTestClient(
 				this.restClientBuilder, this.entityResultConsumer, new DefaultRestTestClientBuilder<>(this));
 	}