By Rodrigo Viga Gaier RIO DE JANEIRO (Reuters) -Presidents of the four Mercosur nations met for their annual summit on Thursday with black clouds hovering over the fate of a trade deal with the European Union, as well as the future of the South American common market itself. They had hoped to announce a free trade agreement with the EU, opening markets home to some 732 million people. The change of government in Argentina just three days after the summit has derailed a final agreement on issues that would need approval from the incoming president, Javier Milei, a radical libertarian who has sa…