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

extensions/resteasy-reactive/rest/deployment/src/test/java/io/quarkus/resteasy/reactive/server/test/providers/WriterInterceptorContentTypeOverrideTest.java

@@ -0,0 +1,59 @@
+package io.quarkus.resteasy.reactive.server.test.providers;
+
+import java.io.IOException;
+
+import jakarta.annotation.Priority;
+import jakarta.ws.rs.GET;
+import jakarta.ws.rs.Path;
+import jakarta.ws.rs.Produces;
+import jakarta.ws.rs.WebApplicationException;
+import jakarta.ws.rs.core.MediaType;
+import jakarta.ws.rs.core.Response;
+import jakarta.ws.rs.ext.Provider;
+import jakarta.ws.rs.ext.WriterInterceptorContext;
+
+import org.hamcrest.Matchers;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.RegisterExtension;
+
+import io.quarkus.test.QuarkusUnitTest;
+import io.restassured.RestAssured;
+
+public class WriterInterceptorContentTypeOverrideTest {
+    @RegisterExtension
+    static final QuarkusUnitTest TEST = new QuarkusUnitTest().withApplicationRoot(
+            jar -> jar.addClasses(MediaTypeResponseInterceptor.class, TestResource.class));
+
+    @Test
+    public void producesMediaTypePresentInWriterInterceptor() {
+        RestAssured.when().get("/test").then().body(Matchers.containsString("Hello"));
+    }
+
+    @Path("/test")
+    public static class TestResource {
+
+        @GET
+        @Produces(MediaType.TEXT_XML)
+        public Response hello() {
+            Greeting greeting = new Greeting("Hello");
+            return Response.ok(greeting).type(MediaType.TEXT_PLAIN).build();
+        }
+    }
+
+    public record Greeting(String message) {
+    }
+
+    @Priority(5000)
+    @Provider
+    public static class MediaTypeResponseInterceptor implements jakarta.ws.rs.ext.WriterInterceptor {
+
+        @Override
+        public void aroundWriteTo(WriterInterceptorContext context) throws IOException, WebApplicationException {
+            if (!context.getMediaType().isCompatible(MediaType.TEXT_PLAIN_TYPE)) {
+                throw new IllegalStateException("MediaType was not overridden by Response, got: " + context.getMediaType()
+                        + " instead of expected: " + MediaType.TEXT_PLAIN);
+            }
+            context.proceed();
+        }
+    }
+}

extensions/resteasy-reactive/rest/deployment/src/test/java/io/quarkus/resteasy/reactive/server/test/providers/WriterInterceptorContentTypeTest.java

@@ -0,0 +1,59 @@
+package io.quarkus.resteasy.reactive.server.test.providers;
+
+import java.io.IOException;
+
+import jakarta.annotation.Priority;
+import jakarta.ws.rs.GET;
+import jakarta.ws.rs.Path;
+import jakarta.ws.rs.Produces;
+import jakarta.ws.rs.WebApplicationException;
+import jakarta.ws.rs.core.MediaType;
+import jakarta.ws.rs.core.Response;
+import jakarta.ws.rs.ext.Provider;
+import jakarta.ws.rs.ext.WriterInterceptorContext;
+
+import org.hamcrest.Matchers;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.RegisterExtension;
+
+import io.quarkus.test.QuarkusUnitTest;
+import io.restassured.RestAssured;
+
+public class WriterInterceptorContentTypeTest {
+    @RegisterExtension
+    static final QuarkusUnitTest TEST = new QuarkusUnitTest().withApplicationRoot(
+            jar -> jar.addClasses(MediaTypeResponseInterceptor.class, TestResource.class));
+
+    @Test
+    public void producesMediaTypePresentInWriterInterceptor() {
+        RestAssured.when().get("/test").then().body(Matchers.containsString("Hello"));
+    }
+
+    @Path("/test")
+    public static class TestResource {
+
+        @GET
+        @Produces(MediaType.TEXT_XML)
+        public Response hello() {
+            Greeting greeting = new Greeting("Hello");
+            return Response.ok(greeting).build();
+        }
+    }
+
+    public record Greeting(String message) {
+    }
+
+    @Priority(5000)
+    @Provider
+    public static class MediaTypeResponseInterceptor implements jakarta.ws.rs.ext.WriterInterceptor {
+
+        @Override
+        public void aroundWriteTo(WriterInterceptorContext context) throws IOException, WebApplicationException {
+            if (!context.getMediaType().isCompatible(MediaType.TEXT_XML_TYPE)) {
+                throw new IllegalStateException("MediaType was not provided, got: " + context.getMediaType()
+                        + " instead of expected: " + MediaType.TEXT_XML);
+            }
+            context.proceed();
+        }
+    }
+}

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();
     }