Leading technology companies said Tuesday that a monthslong breach of corporate and government networks was so sophisticated, focused and labor-intensive that a nation had to be behind it, with all the evidence pointing to Russia. In the first congressional hearing on the breach, representatives of technology companies involved in the response described a hack of almost breathtaking precision, ambition and scope. “We haven’t seen this kind of sophistication matched with this kind of scale,” Microsoft President Brad Smith told the Senate Intelligence Committee.