The 93-year-old widow of a Wall Street financier has donated $1 billion to a Bronx medical school, the Albert Einstein College of Medicine, with instructions that the gift be used to cover tuition for all students going forward. The donor, Dr. Ruth Gottesman, is a former professor at Einstein, where she studied learning disabilities, developed a screening test and ran literacy programs. The fortune came from her late husband, David Gottesman, known as Sandy, who was a protégé of Warren Buffett and had made an early investment in Berkshire Hathaway, the conglomerate Mr. Buffett built. Dr. Gottesman had a long career at Einstein, a well-regarded medical school, starting in 1968, when she took a job as director of psychoeducational services. She has long been on Einstein’s board of trustees and is currently the chair.