DETROIT — A man accused of planting a bomb outside of a Detroit federal building four years ago may never stand trial despite extraordinary efforts to improve his mental health. Two courts allowed the government to forcibly medicate Gary Mikulich, but he’s still not mentally competent for trial. The issue now is whether he will stay locked up indefinitely because of his possible threat to the public. The 2011 incident embarrassed the government. A Husky-brand tool bag holding the explosive was left …