Is the saying narrated in the Qur’an attributed to Allah Almighty, or to its original speaker, such as "Indeed, your cunning is great" or the disapproval of the sound of donkeys?

The rule is that whatever Allah Almighty has narrated in His Book about one of His creations, if He refutes it, it indicates its falsity, and if He does not refute it, it indicates its validity. Al-Shatibi and Ibn 'Uthaymeen have agreed upon this rule.

The speech narrated in the Holy Quran is by meaning, not by literal wording. It is from the speech of Allah, but it is attributed to the one who uttered it based on its content.

Thus, all narrated statements in the Quran are the speech of Allah Almighty. If Allah does not refute the one who uttered it, it is used as evidence for the meanings it contains, such as the saying of Luqman to his son: (And be moderate in your pace and lower your voice; indeed, the most disagreeable of sounds is the voice of donkeys.) [Luqman/19].

However, if Allah refutes it and declares it false on the part of its speaker, it is also said about it: Allah Almighty said, but it is not used as evidence for the meanings it contains. Rather, it clarifies that Allah has invalidated this statement, as in the saying: (Say, "Indeed, Allah does not order immorality.") [Al-A'raf/28].

And (Indeed, your plot is tremendous) [Yusuf/28] is from the speech of Al-Aziz. Allah mentioned it and did not refute it, so it is an affirmation of it. However, it is not generalized to all women, but rather came in the context of specific events.