he existence of such a privilege is based upon a policy of encouraging open- ness and full disclosure between a client and his or her attorney. See Upjohn Co.' United States, 449 U.S. 383, 389, 101 S.Ct. 677, 682, 66 L.Ed.2d 584 (1981); Priest' Hennessy, 51 N.Y.2d 62, 6748, 431 N.Y. S.2d 611, 613-514, 40