What is the ruling on saying: "May Allah bless you, O Messenger of Allah, we supplicate and hope for your intercession on the Day of Gathering," and is the one who says it considered a polytheist?

If the speaker intends to supplicate the Prophet and seek intercession from him, this is major shirk, and it must be clarified that it is not permissible to supplicate anyone other than Allah. However, if the speaker intends to bring the Prophet to mind and address him, and their aim is for Allah Almighty to grant them the Prophet's intercession, then there is no prohibition in this, as it falls under the category of addressing a conceived image within oneself.