BERLIN — Leading German economists warned Chancellor Friedrich Merz on Wednesday to rein in soaring social spending, saying the costs risk plunging Europe’s largest economy into prolonged stagnation. The German Council of Economic Experts, an influential panel of economic advisers, will discuss its economic outlook with Merz later in the day. Their findings are poised to increase pressure on the chancellor, who is already racing to assemble sweeping reforms to tax, health insurance, and pension