SEOUL, South Korea — Russia has supplied air defense missile systems to North Korea in exchange for sending its troops to support Russia's war efforts against Ukraine, a top South Korean official said Friday. The U.S., South Korea and Ukraine say North Korea sent more than 10,000 troops to Russia in October, some of whom have recently begun engaging in combat on the front lines. North Korea's troop deployment threatens to escalate the war, and what Russia could give to the North in return has