Robert A. Urciuoli, convicted of bribing a former state senator, is seeking almost $3.6 million in severance pay and legal costs from his longtime employer, Roger Williams Medical Center, and its new owner. Urciuoli, 67, is demanding arbitration with the medical center and CharterCARE Health Partners, its new for-profit owners. He argues that under his employment contract he could only be terminated "for cause" and that evidence shows that lawyers advised that the agreement he reached with ex-state Sen. John A. Celona to promote the hospital and an associated nursing home complied with the law.