How to test lambda expression included private method using Mockito?

Question

I want to test this code.

public void send(String topic, Object value) {
        Optional.ofNullable(observer.get(topic)).ifPresent(handler::removeCallbacks);
        observer.put(topic, () -> send(topic, value));
        MessageObservable observable = MessageSendingObserver.create((isSuccess, publicationReport){
            if (shouldBeResend(isSuccess, publicationReport)) {
                handler.postDelayed(observer.get(topic), INTERVAL);
            }
        });
        message.publish(topic, value, observable);
    }
    
private boolean shouldBeResend(boolean isSuccess, PublicationReport publicationReport) {
        return !isSuccess && publicationReport.getErrorCode() >= 400 && publicationReport.getErrorCode() <= 599;
} 
I want to test this code.

I have problem how to mock observer and force it to throw error or return value on success. I'm using mockito/junit. Can someone point me how to achieve it? Maybe my code is untestable?