MOSCOW — When Vladimir Putin jumped into the ring to congratulate the winner of a bloody martial arts fight and was greeted with a cascade of boos from a large part of the 20,000-plus crowd, the experience seemed just this once to set him back on his heels, if only for a moment. Throughout much of the world, politicians can expect to provoke the boo-birds when they show up at sports events, but Sunday evening’s bout — shown live on national television — was a first for Russia’s authoritarian prime minister.