Are the prayers considered invalid if the worshiper recites Al-Fatiha with "Ihdina al-sirat al-mustaqim" instead of "Ihdina al-sirat al-mustaqim"?

If you read "Ihdina" (اهدنا - Guide us) with a kasra on the hamza when starting with it after pausing on "nasta‘in" (نستعين - we ask for help), this is correct. However, if you read it with a kasra on the hamza when connecting it to "nasta‘in" without pausing, this is a laḥn (mistake in recitation) but does not invalidate the prayer because it does not change the meaning. This is because the hamzat al-waṣl (connecting hamza) is dropped during continuous recitation and is pronounced only when starting with the word.