Brian Steinberg Senior TV EditorA fight for answers as to why their children were battling rare cancers forms the crux of “In the Dark of the Valley,” the newest acquisition by MSNBC Films, a nascent unit that hopes to build the cable-news network’s pipeline of longform projects.The documentary tells the story of a mother in southern California who finds that an abandoned rocket-testing facility, called the Santa Susana Field Laboratory, near her home was the site of one of the largest nuclear