Fix serialisation in spring-boot 4 for restclient (#23588)

Author: alex-nt

modules/openapi-generator/src/main/resources/Java/libraries/restclient/ApiClient.mustache

@@ -239,9 +239,9 @@ public class ApiClient{{#jsr310}} extends JavaTimeFormatter{{/jsr310}} {
 
         {{#useJackson3}}
         Consumer<HttpMessageConverters.ClientBuilder> messageConverters = builder -> {
-            builder.withJsonConverter(new JacksonJsonHttpMessageConverter(mapper));
+            builder.addCustomConverter(new JacksonJsonHttpMessageConverter(mapper));
             {{#withXml}}
-            builder.withXmlConverter(new JacksonXmlHttpMessageConverter(xmlMapper));
+            builder.addCustomConverter(new JacksonXmlHttpMessageConverter(xmlMapper));
             {{/withXml}}
         };
 

samples/client/petstore/java/restclient-springBoot4-jackson3-jspecify/src/main/java/org/openapitools/client/ApiClient.java

@@ -148,7 +148,7 @@ protected void init() {
     public static RestClient.Builder buildRestClientBuilder(JsonMapper mapper) {
 
         Consumer<HttpMessageConverters.ClientBuilder> messageConverters = builder -> {
-            builder.withJsonConverter(new JacksonJsonHttpMessageConverter(mapper));
+            builder.addCustomConverter(new JacksonJsonHttpMessageConverter(mapper));
         };
 
         return RestClient.builder().configureMessageConverters(messageConverters);

samples/client/petstore/java/restclient-springBoot4-jackson3/src/main/java/org/openapitools/client/ApiClient.java

@@ -150,7 +150,7 @@ protected void init() {
     public static RestClient.Builder buildRestClientBuilder(JsonMapper mapper) {
 
         Consumer<HttpMessageConverters.ClientBuilder> messageConverters = builder -> {
-            builder.withJsonConverter(new JacksonJsonHttpMessageConverter(mapper));
+            builder.addCustomConverter(new JacksonJsonHttpMessageConverter(mapper));
         };
 
         return RestClient.builder().configureMessageConverters(messageConverters);