During his glittering career, George Clooney has played a casino thief, a Batman, a chain-gang convict, an assassin and a high-flying layoff artist. This fall, he’s stretching even more, playing an utterly charming and gorgeous movie star. Kidding! Reality and fiction beautifully weave in and out in “Jay Kelly,” director Noah Baumbach’s love letter to Hollywood that, in other hands, could so easily have become just a love letter to Clooney. The script by Baumbach and Emily Mortimer finds