verify(smbFile, times(1)).isDirectory();
        verifyNoMoreInteractions(smbFile);
    }

    @Test
    @DisplayName("accept: rejects null input with meaningful NullPointerException")
    void accept_rejectsNullInput() {
        // Arrange: defensive filter that validates input
        SmbFileFilter filter = f -> {
            if (f == null) {
                throw new NullPointerException("file must not be null");