By Natalia Zinets MARIUPOL, Ukraine (Reuters) -More than 250 Ukrainian fighters surrendered to Russian forces at the Azovstal steelworks in Mariupol after weeks of desperate resistance, bringing an end to the most devastating siege of Russia’s war in Ukraine and allowing President Vladimir Putin to claim a rare victory in his faltering campaign. Even as the Kremlin prepares to take full control of the ruins of Mariupol, it faces the growing prospect of defeat in its bid to conquer all of Ukraine’s eastern Donbas because its badly mauled forces lack the manpower for significant advances, some a…