Ariad Pharmaceuticals Inc. is set to disclose today that it has won a key patent on its drug to treat a form of leukemia, giving the Cambridge biotechnology company patent protection through 2026 on what could become a major cancer-fighting therapy. Shares of Ariad have more than doubled in price over the past year, climbing sharply since the start of 2012, amid projections that ponatinib, its treatment for chronic myeloid leukemia, or CML, could generate sales of more than $1 billion.