BEIJING — China’s Xi Jinping warned President Donald Trump on Thursday that their two countries could clash over Taiwan if the issue is not handled properly, an unusually harsh admonition that stood in contrast to the American leader’s praise for his counterpart. The exchange at a highly anticipated summit in Beijing underscored just how far apart Trump and Xi still are on thorny issues, including the war in Iran, trade disputes and Washington’s relations with Taiwan, which is self-ruled but which China claims as part of its territory. It also suggested that Trump’s three-day visit to China is likely to be longer on pageantry and symbolism than substantive political or economic breakthroughs. The pair met for about two hours behind closed doors at the Great Hall of the People after an elaborate welcome ceremony featuring booming cannons, a band playing “The Star-Spangled Banner” and China’s national anthem, and hundreds of schoolchildren jumping and waving flowers and American and Chinese flags.…