Federal prosecutors rebuffed a judge’s order to release by Friday highly classified transcripts of discussions that Michael T. Flynn, the president’s former national security adviser, had with the Russian ambassador during the presidential transition. The transcripts between Mr. Flynn and Sergey I. Kislyak, formerly Russia’s top diplomat in the United States, were expected to show that they talked in December 2016 about sanctions that the Obama administration had just imposed on Russia. Mr. Flynn initially denied those exchanges about sanctions both to Trump administration officials and the F.B.I. in the weeks after the discussions. The conversations prompted concerns among senior...