HEEEEERE'S HENRI: With Johnny Carson gone from "The Tonight Show," what kind of guests will new host Jay Leno come up with? How about Henri Venanzi, chorus master for the Costa Mesa-based Opera Pacific? Actually, he won't be center stage. On the May 29 show, he'll play piano for the Metropolitan Opera's Kathleen Battle and Grammy-winning saxophonist Wynton Marsalis. . . . Venanzi recently made a hit with Battle when he accompanied her in Kansas City as a last-minute replacement.