The Justice Department sided with the State Department against attempts by a court-appointed federal prosecutor to force Canada's ambassador to the United States to testify against former White House aide Michael K. Deaver at his forthcoming perjury trial. Justice Department attorneys said in court papers that independent counsel Whitney North Seymour Jr. had overstepped his bounds in sending Canada a letter threatening that, unless Ambassador Allan Gotlieb abandoned his diplomatic immunity and became a government witness, U.S. prosecutors "will be forced . . . to place much greater emphasis at trial on the unlawful acts engaged in by Deaver when he was working for the Canadian government."