Just three months ago, China’s top leader, Xi Jinping, was in Moscow clinking glasses with Vladimir Putin and expressing his confidence in the “firm support” the Russian president enjoyed among his people. That confidence is now in question, after the Wagner private military group waged an insurrection in Russia that has shaken Putin’s image of invulnerability. Close watchers of China say that the mutiny, short-lived as it was, could lead Xi to hedge a close relationship with Russia that had exposed Beijing to global criticism and threatened some of its interests abroad. China views Russia as a necessary partner in...