As Iran’s clerical rulers face one of the biggest threats to their grip on power in years and the United States threatens potential military action in support of Iranian protesters, China has few options -- and growing hesitancy -- to help its partner in Tehran. Beijing’s inaction reflects the pragmatic nature of their alignment, which is based more on convenience rather than goodwill or trust, experts say. Those limits have been on display as the Iranian authorities wage an unprecedented and bloody crackdown on mass antiestablishment…