The California Supreme Court on Thursday upheld Proposition 22, the voter initiative that allows Uber, Lyft and other gig economy companies to classify drivers for their ride-hailing and delivery services as independent contractors rather than as employees. In a unanimous decision released Thursday morning, the state’s top court rejected claims brought by a group of drivers and unions that the law is unconstitutional because it interferes with lawmakers’ authority over matters dealing with worker compensation. The ruling, which was expected following a lopsided hearing in the case in May, marks the end of a years-long legal fight over Proposition 22,...