Use proper media type when executing writer interceptors

The one from the response can be null if we return a partial response
but the type is defined by @produces.
We need to consider the one from the context.

Fixes #34839

independent-projects/resteasy-reactive/server/runtime/src/main/java/org/jboss/resteasy/reactive/server/core/ServerSerialisers.java

@@ -262,8 +262,8 @@ public static boolean invokeWriter(ResteasyReactiveRequestContext context, Objec
     public static void runWriterInterceptors(ResteasyReactiveRequestContext context, Object entity, MessageBodyWriter writer,
             Response response, WriterInterceptor[] writerInterceptor, ServerSerialisers serialisers) throws IOException {
         WriterInterceptorContextImpl wc = new WriterInterceptorContextImpl(context, writerInterceptor, writer,
-                context.getAllAnnotations(), entity.getClass(), context.getGenericReturnType(), entity, response.getMediaType(),
-                response.getHeaders(), serialisers);
+                context.getAllAnnotations(), entity.getClass(), context.getGenericReturnType(), entity,
+                context.getResponseContentType().getMediaType(), response.getHeaders(), serialisers);
         wc.proceed();
     }