BEIJING — A top Chinese military officer, in a rare meeting with a visiting American official, demanded Thursday that the United States stop “collusion” with Taiwan, the self-governing island that China says must come under its rule. Gen. Zhang Youxia, one of two vice chairs of the Central Military Commission, told White House National Security Adviser Jake Sullivan that promoting what China calls the reunification of Taiwan with the mainland is “the mission and responsibility” of the military,