WASHINGTON — Earlier this summer, President Joe Biden was feeling hopeful. His son Hunter Biden’s lawyers had struck a plea deal with federal prosecutors on tax and gun charges, and it seemed to the president that the long legal ordeal would finally be over. But when the agreement collapsed in late July, the president, whose upbeat public image often belies a more mercurial temperament, was stunned. Sign up for The Morning newsletter from the New York Times He plunged into sadness and frustratio