When China hatched the first of a series of oil-for-loans agreements with Venezuela in 2007, it seemed like a perfect match. Venezuela had the world’s biggest oil reserves; China was poised to become the biggest energy consumer. Twelve years and more than $50 billion in loans later, a political crisis in Venezuela is threatening China’s payout and drawing Beijing into a proxy standoff as it supports a Venezuelan leader the U.S. is intent on toppling. It is a conflict with Washington that Beijing could do without, amid…