BERLIN — Germany marked a quarter-century as a reunited nation on Saturday, with two leaders from the formerly communist east heading a country that increasingly asserts itself as Europe’s political heavyweight — and now faces a new challenge in a refugee influx that will demand deep reserves of resourcefulness and patience. [...] “things worked out well — so many people pitched in, showed verve, began to learn new jobs,” Chancellor Angela Merkel, who grew up in the east and entered politics as communism fell, said in a video message ahead of the anniversary. Since reunification, some 1.5 trillion to 2 trillion euros (up to $2.2 trillion) have been funneled into the east to help modernize the region after its outmoded industry collapsed.