WASHINGTON — The American job market has defied raging inflation, rising interest rates, growing recession fears. Month after month, U.S. employers just kept adding hundreds of thousands of workers, often beating forecasters’ expectations. But now economists worry that signs of weakness are starting to turn up in hiring, threatening one of the United States’ last remaining redoubts of economic strength. Job openings are down, and the number of Americans signing up for unemployment benefits is up.