Is the prayer of one who says "Sami'a" instead of the bowing dhikr, "Subbuh Quddus, Rabb al-Mala'ikati wa al-Ruh," during prostration, and does not rise until after saying the dhikr, valid?

Your saying "Sami'" (He heard) by mistake is considered a verbal addition, and there is a difference of opinion among scholars regarding the legality of performing the prostration of forgetfulness (sujood as-sahw) for it. If it is legislated, it is only recommended, not obligatory, and there is no sin in omitting it. The prayer is not invalidated by it. Your prayer is valid, and there is nothing upon you.