State Senator Michael F. Rush has taken the idea of public service to a new level. Kathleen Joyce, a Dorchester resident, said in an interview that she was on Beacon Hill Dec. 12 when a man snatched her iPhone and ran away. Rush, a West Roxbury Democrat whose district includes Norwood, saw the theft, got out of his car, and chased the man, who dropped the phone before disappearing over a fence, Joyce said. Rush returned the phone to Joyce, who called the senator “a very good Samaritan.”