They have met more than a dozen times and stood shoulder to shoulder during Thursday’s military parade here. But the once-vaunted relationship between the Chinese president, Xi Jinping , and Russia ’s leader, Vladimir V. Putin , has come under strain as the economies of their countries have faltered. Two landmark energy deals signed last year for Russian natural gas to flow to China have made little progress and were barely mentioned when the two men met for talks after watching the show of weapons Thursday on Tiananmen Square. The bilateral trade that was predicted to amount to more than $100 billion this year instead reached only about $30 billion in the first six months, largely because of a reduced Chinese demand for Russian oil […]