Prevent REST Client handling of abstract classes

Fixes: quarkusio#42977

independent-projects/resteasy-reactive/common/processor/src/main/java/org/jboss/resteasy/reactive/common/processor/scanning/ResteasyReactiveScanner.java

@@ -324,7 +324,7 @@ public static ResourceScanningResult scanResources(
                     MethodInfo annotatedMethod = methodAnnotationInstance.target().asMethod();
                     ClassInfo classWithJaxrsMethod = annotatedMethod.declaringClass();
                     if (Modifier.isAbstract(annotatedMethod.flags())
-                            && Modifier.isAbstract(classWithJaxrsMethod.flags())
+                            && Modifier.isInterface(classWithJaxrsMethod.flags())
                             && !clientInterfaces.containsKey(classWithJaxrsMethod.name())) {
                         clientInterfaces.put(classWithJaxrsMethod.name(), "");
                     }

integration-tests/rest-client-reactive-http2/src/main/java/io/quarkus/it/rest/client/http2/AbstractGreetingResource.java

@@ -0,0 +1,14 @@
+package io.quarkus.it.rest.client.http2;
+
+import jakarta.ws.rs.GET;
+import jakarta.ws.rs.Path;
+import jakarta.ws.rs.Produces;
+import jakarta.ws.rs.core.MediaType;
+
+@Path("/greet")
+public abstract class AbstractGreetingResource {
+
+    @GET
+    @Produces(MediaType.TEXT_PLAIN)
+    public abstract String hello();
+}

integration-tests/rest-client-reactive-http2/src/main/java/io/quarkus/it/rest/client/http2/GreetingResource.java

@@ -0,0 +1,8 @@
+package io.quarkus.it.rest.client.http2;
+
+public class GreetingResource extends AbstractGreetingResource {
+    @Override
+    public String hello() {
+        return "Hello";
+    }
+}

integration-tests/rest-client-reactive-http2/src/test/java/io/quarkus/it/rest/client/http2/GreetingResourceTest.java

@@ -0,0 +1,19 @@
+package io.quarkus.it.rest.client.http2;
+
+import static io.restassured.RestAssured.when;
+
+import org.junit.jupiter.api.Test;
+
+import io.quarkus.test.junit.QuarkusTest;
+
+@QuarkusTest
+class GreetingResourceTest {
+
+    @Test
+    void testGreeting() {
+        when()
+                .get("/greet")
+                .then()
+                .statusCode(200);
+    }
+}