LONDON — Labour MP Wes Streeting insisted on Tuesday that he has secured the backing of fellow MPs that he would need to fight a contest to be Britain’s next prime minister. The former health secretary said he wants a “genuine contest” to decide who will replace Keir Starmer in Downing Street, not an unopposed coronation for Andy Burnham — the Greater Manchester mayor, who is the favorite for Starmer’s job if he wins a crunch parliamentary by-election this Thursday. But while insisting he has