Litigation challenging the North Carolina General Assembly’s decision a decade ago to charge some retired state government workers and teachers a premium for health insurance is now before the state Supreme Court. Oral arguments are set for Monday. The court’s ultimate decision could affect hundreds of thousands of retirees and cost the state premium refunds and future health care expenses. The plaintiffs argue the state was contractually obligated to offer premium-free benefits under a plan in which they paid 20 percent of their co-insurance. A trial court agreed in 2017, but the Court of Appeals overturned that decision two years later.