Are the jinn who embraced Islam during the time of the Prophet, peace and blessings be upon him, considered among the Companions?

Yes, the Jinn are considered among the Companions (Sahabah), because the definition of a Companion is one who met the Prophet Muhammad, peace be upon him, believed in him, and died upon that faith. The Jinn met the Prophet, peace be upon him, and believed in him, so they fall under the designation of companionship, as mentioned in the Holy Quran in Surah Al-Jinn and Surah Al-Ahqaf. Ibn Hazm and Ibn Hajar affirmed that the Jinn who met the Prophet and believed in him hold the same status as the Companions.