BEIJING — China’s economic growth sank to its lowest level in at least 26 years in the quarter ending in June, adding to pressure on Chinese leaders as they fight a tariff war with Washington. The world’s second-largest economy grew 6.2 percent over a year ago, down from the previous quarter’s 6.4 percent, government data showed Monday. Forecasters expected China’s economy to rebound in late 2018 but pushed back that target after President Donald Trump raised tariffs on Chinese imports to pressure Beijing over its technology development tactics.