/**
     * Simulate a witness unregistration
     *
     * @param registrationId the registration ID to remove
     * @return true if registration was found and removed
     */
    public boolean unregisterWitness(String registrationId) {
        MockRegistration removed = registrations.remove(registrationId);
        if (removed != null) {
            log.debug("Mock unregistered witness: {}", registrationId);