Does covering my nakedness in front of a worker who performed a full-body bath (hammam shamil) necessitate an expiation (kaffarah) or anything else, knowing that I regret my action and have sworn not to return to it?

What you did by exposing your nakedness is a reprehensible act, and you should make your remorse for it sincere repentance. As for the expiation for an oath, if you return to massage, and you intended to prevent yourself from massages that involve reprehensible acts, then there is no expiation upon you if you practice massage according to Sharia guidelines. There is no harm in performing massage in public places if they are free from reprehensible acts, but it is better to avoid them if there is a possibility of reprehensible acts. Let those who need massage perform it with their family members. It is not permissible for a Muslim to supplicate for death and destruction upon themselves.