Rachel McAdams brings an instantly heartbreaking quality to her performance in “Mary Jane”: Her steadfast optimism. Writer Amy Herzog’s affecting play, which opened Tuesday night on Broadway at the Samuel J. Friedman Theatre, is about a single mother whose two-year-old son Alex is chronically ill. The depth that McAdams gives Mary Jane, in the most natural way, is her positivity. In the film “The Notebook,” in the TV series “Slings & Arrows,” even as the villain in the movie “Mean Girls,” the actress has always emanated a light from within.