Jay Leno played Cupid last night on the "Tonight Show," helping to engineer a live TV proposal. The happy couple? Model-turned actress Angie Harmon ("Law & Order") and New York Giants cornerback Jason Sehorn. Harmon, 27, was a scheduled guest last night on the late night talk show, where Leno questioned her about her love life. When Harmon evaded Leno's queries, he surprised her by bringing Sehorn, 28, on stage. On bended knee, and with a diamond ring in hand, Sehorn popped the question to Harmon. (She said yes: "Jason . . . oh, God . . . baby, yes!") Also there to witness the romantic moment was Harmon's father and musical guest Elton John, who sang a song from the movie "The Road to Eldorado" for the couple. This will be Harmon's first marriage; Sehorn divorced his first wife last March.