ISTANBUL — Russia and Turkey agreed Tuesday on a plan to push Syrian Kurdish fighters from a wide swath of territory just south of Turkey’s border, cementing Russian President Vladimir Putin’s preeminent role in Syria as U.S. troops depart and America’s influence wanes.More important, though, the deal bolstered Russia’s preferred endgame in Syria’s civil war by allowing its ally, Syrian President Bashar al-Assad, to regain control over more of his country’s territory. Russia is also prodding states in the region to recognize, either explicitly or tacitly, the Syrian government’s authority, analysts said.