Zohran Mamdani had a significant lead over former Gov. Andrew Cuomo in New York City’s Democratic mayoral primary Tuesday night, leaving the young liberal upstart poised to land a stunning upset in a race still to be determined by a ranked-choice count. Cuomo, trying to make a comeback from a sexual harassment scandal, conceded the contest less than two hours after the polls closed, saying he had called Mamdani to congratulate him.