/**
     * 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);

        // Unregister one
        assertTrue(mockService.unregisterWitness(reg2));
        assertEquals(2, mockService.getRegistrationCount());
        assertFalse(mockService.hasRegistration(reg2));

        // Clean up remaining
        assertTrue(mockService.unregisterWitness(reg1));
        assertTrue(mockService.unregisterWitness(reg3));
        assertEquals(0, mockService.getRegistrationCount());
    }