Holding the key to the sale of America’s second-biggest candy company is a scandal-scarred, $12 billion charity that all but owns Hershey, Pennsylvania. Hershey Trust Co. controls about 80 percent of Hershey Co., guides the 107-year-old Milton Hershey School, and oversees an amusement park and resort in the town of about 14,000. Its 10 trustees have been averse to deals, scuttling efforts to separate them from their candy-coated source of cash. The Hershey Co. board rejected Mondelez International Inc.’s $23 billion bid Thursday to put together the biggest candy maker in the world by buying the ailing chocolate company. What surprised some observers was that Hershey offered no rationale for rebuffing Mondelez, saying only that it saw nothing in the offering that warranted further discussion.