KYIV, Ukraine — Russia’s Wagner mercenary group has been forced to use more of its professional recruits in Bakhmut to replace its depleted supply of enlisted prisoners, who are perishing by the thousands in the longest battle of the war, a Ukrainian official said on Tuesday. The claim suggested that Ukraine sees an opportunity, despite the heavy casualties it has suffered in the eastern city, to exhaust Wagner’s nearly suicidal prisoner assaults, which Ukraine’s commanders regard as one of Russia’s most effective tactics. “This is their last stand,” Col. Serhiy Cherevaty, a spokesperson for Ukraine’s eastern group of forces, told...