WASHINGTON — A federal judge on Tuesday upheld an obstruction conviction against a Virginia man who stood trial with members of the Oath Keepers extremist group in one of the most serious cases brought in the Jan. 6, 2021, insurrection.U.S. District Judge Amit Mehta rejected a defense effort to throw out the Washington jury’s guilty verdict against Thomas Caldwell, a retired U.S. Navy intelligence officer who was convicted last November in the U.S. Capitol attack alongside Oath Keepers founder Stewart Rhodes …