WASHINGTON — U.S. Food and Drug Commissioner Robert Califf told Congress Thursday that the infant formula shortage will likely begin to improve in the coming days, though he declined to explain to unhappy lawmakers why his agency didn’t act sooner on a whistleblower report that detailed safety issues at an Abbott Laboratories facility in Michigan. “We have an ongoing investigation about the details of exactly what happened, you know, from point A to point B along the way,” Califf said at a tense U.S. House hearing.