Tesla Inc. ended last year on a roll, with investors increasingly buying into Elon Musk’s ebullience about autonomous vehicles. Winning over actual car buyers was another story.Shares in the world’s most valuable auto company soared in the second half, largely on the basis of its chief executive officer touting advances in artificial intelligence and robotics. But the progress Musk trumpeted didn’t translate to success in showrooms — the company likely sold fewer vehicles in the last six months than a year ...