What is the authenticity of the hadith mentioned on the authority of Ka’b al-Ahbar concerning the description of Paradise and the bliss of its inhabitants and their vision of the Lord on the Day of Eid?

The Hadith is what has been attributed to the Prophet in terms of speech, action, approval, or a physical or moral description. What was mentioned in the question is not a Hadith, because it is not attributed to the Prophet. Rather, it is a statement attributed to Ka'b al-Ahbar, who was one of the Tabi'un (Successors) scholars and narrated from the books of the People of the Book. It is not authentically established from Ka'b, because its chain of transmission includes Yazid ibn Abi Ziyad, who is weak.