Russia is sending reinforcements to Libya to help military leader Khalifa Haftar, who is on the defensive after a failed effort to topple the country’s United Nations-backed government, as Moscow seeks to shape the fate of the nation and its vast oil reserves. Private military contractors from Russia helped Mr. Haftar’s forces take control of Libya’s largest oil field last week, according to European and Libyan officials. Russian cargo planes have regularly shuttled between a Russian air base in Syria and Libya in recent…