Career Reborn, on a High Note
Bejun Mehta, onetime boy soprano who is now 30, discusses debut as countertenor in Handel's Partenope at New York City Opera; interview; photos; Mehta, cousin of Zubin Mehta, was born in Ann Arbor to musical family, and was child prodigy; he later took up cello and studied Germanic literature at Yale; Marilyn Horne, his most influential supporter, comments on his unsuccessful efforts as baritone (M)