What is the ruling on extreme fear of the jinn while observing prayers, and is it considered polytheism, given that the person believes that Allah alone is the One Who harms and benefits?

Your fear of what might harm or hurt you is not considered shirk (polytheism) unless you intend to glorify it or believe that it possesses benefit or harm inherently. You must rid yourself of this intense fear by embracing courage, trusting in Allah, and relying on Him. You should also dispel the illusions of fear and resist the whispers of Shaytan (evil suggestions), supplicate to Allah, and seek refuge in Him from the jinn, with the certainty that they possess neither harm nor benefit. Furthermore, maintain the prescribed remembrances (adhkar) such as those for morning, evening, and before sleep, and frequently recite the Qur'an, especially Surat Al-Baqarah, Ayat Al-Kursiyy, and the two Mu'awwidhatayn (Surat Al-Falaq and Surat An-Nas). Increase your remembrance of Allah, for it is a protection from evil.