What is the ruling on someone who forgot the two prostrations of forgetfulness (Sajdat As-Sahw), then remembered them while still in place after the Witr prayer and performed them?

Your prayer is valid and there is nothing upon you. The jurists mentioned that whoever forgets the prostration of forgetfulness (sujud (definition) al-sahw) until he begins another prayer, he should perform it after the salutation of that prayer, as long as the interval has not been long. If the interval has become long, then the prostration is waived, and his prayer is valid. You prostrated after the Witr prayer before a long interval, thus you did what is prescribed.