Artificial intelligence has fired the stock market to record highs this year, with companies eager to tout their AI prowess and investor darlings like AI chipmaker Nvidia soaring  on expectations of runaway growth.  But a tinge of fear is starting to shadow that exuberance as investors worry the AI boom could go bust.  The startling run-up in AI-related stocks is prompting comparisons to the dot-com era of the late 1990s, when many internet companies saw their stock prices skyrocket despite