Tesla aims to stun investors Thursday night with its long-awaited “robotaxi unveil," a potential milestone after a decade of Elon Musk’s unfulfilled promises to deliver self-driving vehicles. The automaker is widely expected to showcase a prototype called the "Cybercab" rather than a road-ready driverless taxi. Convincing regulators and passengers of the vehicle’s safety could prove much harder and take much longer — while its main competitors, such as Alphabet’s Waymo, expand...