WASHINGTON — Powerful public-sector unions are facing another high-profile legal challenge that they say could wipe away millions from their bank accounts and make it tougher for them to survive.A group of California schoolteachers, backed by a conservative group, has asked the Supreme Court to rule that unions representing government workers can’t collect fees from those who choose not to join.Half the states currently require state workers represented by a union to pay “fair share” fees that cover bargaining costs, even …