SEOUL, South Korea—After winning a bitterly contested presidential election, South Korean conservative Yoon Suk Yeol will enter office facing a quickly growing North Korean nuclear threat—and with few easy choices ahead to deal with it. A former prosecutor with no foreign policy experience who kickstarted his political career nine months ago, Yoon will face a turbulent moment in global affairs and the decades-old standoff with the North, over which many experts see Seoul as having lost leverage under the policies of outgoing President Moon Jae-in. It appears Yoon will be tested quickly, possib…