A former F.B.I. informant accused of fabricating a claim that President Biden and his son Hunter were each paid a $5 million bribe by a Ukrainian oligarch has agreed to plead guilty to a range of federal charges, according to a court filing on Thursday. Alexander Smirnov, a profiteer, fixer and gossip based in Las Vegas, reached a deal with the special counsel overseeing the investigation into Hunter Biden, David C. Weiss, that could lead to 48 to 72 months in prison, according to the filing in federal court in California.