MEXICO CITY — Migrants were streaming across the U.S. southern border in record numbers, international rail bridges were abruptly shut down and official ports of entry closed. Desperate for help in December, President Joe Biden called President Andrés Manuel López Obrador of Mexico, who told him to quickly send a delegation to the Mexican capital, according to several U.S. officials. The White House rushed to do so. Soon after, Mexico beefed up enforcement. Illegal border crossings into the Unit