China’s leader, Xi Jinping, flew into Moscow this week cast by Beijing as its emissary for peace in Ukraine. His summit with President Vladimir V. Putin of Russia, however, demonstrated that his priority remains shoring up ties with Moscow to gird against what he sees as a long campaign by the United States to hobble China’s ascent. Talk of Ukraine was overshadowed by Mr. Xi’s vow of ironclad solidarity with Russia as a political, diplomatic, economic and military partner: two superpowers aligned in countering American dominance and a Western-led world…