OMAHA, Neb. — In a surprise noon press conference, Sen. Chuck Hagel announced he has not yet decided whether to audition for next season’s American Idol. But he refused to rule out auditioning, and then sang a few bars of Bobby Brown’s “It’s My Prerogative” in a pleasing baritone. He also said he had not yet made up his mind about whether to have the Penne a la Vodka or a tuna-salad sandwich on whole wheat for lunch.