What should one do if subjected to injustice, mistreatment, and discrimination by their boss at work for refusing to report on colleagues, and is it permissible to pray for revenge in such a situation?

You are permitted to strive to remove injustice from yourself as much as you are able, seeking help from Allah the Almighty and from colleagues who can assist you. There is no sin in supplicating against an oppressor to the extent of his oppression. However, if the supplication exceeds this limit, it is not permissible, and forgiveness is closer to piety.