What is the ruling on the prayer of someone who prayed two rak'ahs, then another person completed it and performed the prostration of forgetfulness?

What you did by stepping forward and completing the prayer with the people is correct, and your prayer is valid. This is because there was an excuse that prevented the Imam from completing his prayer with you, making it permissible for you to appoint someone to complete the prayer with you. A condition for the validity of your stepping forward is that you must not have invalidated your prayer through intentional speech.