WASHINGTON — In mid-July, many of the country’s biggest military contractors received a letter from the Air Force with a stern warning: By Sept. 1, all the software they use for weapons and control systems must be free of products built by the artificial intelligence company Anthropic. Failure to comply would put all their business with the Pentagon at risk.Within a month, those same contractors received an unexpected reversal: They could — for now — disregard the earlier instructions about purging Anthropic.It was one more example of the chaos and contradictions in the tsunami of AI disruptions that has swept…