Is the prayer of one who, by mistake, said the Taslim after the first Tashahhud, then performed the third Rak'ah, and recited the Tashahhud, and performed the prostrations of forgetfulness (sujud al-sahw) before the Taslim, valid?

Scholars have agreed on the validity of prayer, whether the worshiper performs the prostration of forgetfulness (sujud al-sahw) before or after the salutation (taslim); they only differed on what is most preferable. In the described scenario, the more preferable option, according to the majority of scholars, is to prostrate after the salutation, in contrast to the Shafi'is, who hold that it is best for all prostrations of forgetfulness to be before the salutation. Based on this, the prayer is valid, and there is nothing upon the worshiper.