The state is poised for the first time to mandate a minimum level of nurse staffing in hospitals, reigniting a debate over patient safety and health care costs. The Health Policy Commission, the agency that monitors medical costs and related issues, will vote Wednesday on regulations requiring hospitals to staff intensive care units so that each registered nurse is responsible for no more than two patients. If approved, the rules would implement a law passed last year. But a labor union, the 23,000-member Massachusetts Nurses Association, says the proposed rules give hospitals more discretion to assign two patients per ICU nurse than the law intended.