Quarkus Kafka KStream mocking

Question

I have a use case where we have to consume from a kafka stream and produce via Producer Client API (To consume from one cluster and produce on another, and we can only use streams to consume)

@Produces
  public Topology buildTopology() {
    KStream<String, CustomSchema> result =
        streamBuilder.stream(
            "topic", Consumed.with(Serdes.String(), customSerde()));
    result.foreach(
        (key, CustomSchema) -> {
          CustomSchema modifiedResult = #DO_SOMETHING_AND_MODIFY_RECEIVED_DATA;
          // Produce it using Kafka Producer API to another topic of different cluster
          kafkaProducerClient.produce(modifiedResult);
        });
    return streamBuilder.build();
  }

Now, it is working as expected. However, for Unit Testing this (Coverage part mainly, Integration is covered via KafkaCompanion.) (Also, explored TestDriver and TestTopology, it does not give coverage)

I am unable to return an element of type KStream<String, CustomSchema> so that forEach method runs. Currently, post mocking, forEach should have atleast one method to run.

How to return a KStream<String, CustomSchema> with an element for forEach method to run?