NEWTOWN, Penn. — John Fetterman, the state lieutenant governor, has won Pennsylvania’s high-voltage race for an open Senate seat, defeating celebrity TV doctor Mehmet Oz, bringing an end to one of the nastiest and most expensive campaigns of the year as both parties treated it as a potential tipping point for control of the chamber. Fetterman will succeed Sen. Pat Toomey, a Republican who opted against seeking re-election.