What is the legal classification of the mentioned contract—Is it manufacturing (Istisna‘), leasing (Ijarah), or forward sale (Salam)? Does the approximate determination of the term affect its validity, and what are the consequences of its invalidity?

Your transaction is correct, there is no problem with it. You bought the frame and entered into a valid manufacturing contract (istisna') with the optician for the medical lenses. The term ( أجل ) is established by your agreement to deliver tomorrow. The condition of specifying a term in a manufacturing contract is a point of contention among scholars; some prohibit specifying it, while others require it.