By CHRIS MEGERIAN WASHINGTON — It was almost time for the presidential debate, but Kamala Harris’ staff thought there was one more thing she needed to know. So less than an hour before the vice president left her Philadelphia hotel, two communications aides got her on the phone for one of the strangest briefings of her political career. They told her that Donald Trump had been posting on social media about a false and racist rumor that Haitian migrants in Springfield, Ohio, were eating people’s pets.