SEOUL, South Korea—South Korea’s president on Monday urged the incoming Biden administration to build upon the achievements and learn from the failures of President Donald Trump’s diplomatic engagement with North Korea. A dovish liberal and the son of northern war refugees, Moon Jae-in had lobbied hard to help set up Trump’s three summits with North Korean leader Kim Jong Un, but their diplomacy stalemated over disagreements over easing crippling U.S.-led sanctions for the North’s disarmament. Biden has accused Trump of chasing the spectacle of summits rather than meaningful curbs on the North’s nuclear capabilities. North Korea has a history of staging weapons tests and other provocations to test new U.S. presidents, and Kim vowed to strengthen his nuclear weapons program in recent political speeches that were seen as aimed at pressuring an incoming Biden administration. The South Korean leader has been desperate to keep alive a positive atmosphere for dialogue …