BEIJING -- As Presidents Xi Jinping and Donald Trump prepare for a highly anticipated summit , both China and the United States say their ties have been broadly stable in recent months — and they are planning on keeping it that way. But many issues are at stake in one of the world’s most consequential relationships, with no easy end in sight. Few expect major breakthroughs to the long-running frictions between China and the U.S., which range from competition in technology to the thorny question of Taiwan, whose main ally is the U.S. Ending the war with Iran is likely to be added to the agenda, with Beijing being one of the unofficial mediators in the conflict . “On both sides there is a consensus that U.S.-China stability is important,” said Henrietta Levin, senior fellow for the Freeman Chair in China Studies at the Center for Strategic and International Studies in Washington.…