As the first former U.S. senator to be subpoenaed by Congress in more than a century, Jon Corzine testified Thursday about the “last chaotic days” of MF Global, the trading firm that declared bankruptcy under his watch. Corzine said he was “stunned” to learn that the firm could not locate hundreds of millions of dollars in client money in the days before the firm’s collapse, and said he had no idea where the money had gone.