Is saying "I will come, God willing" (sa'āti in sha'a Allah) with the intention of not going considered a lie, and is expiation (kaffarah) obligatory for one who swore by "By God, I will surely come, God willing" (wallahi la'ātiyanna in sha'a Allah) and then did not go?

If one did not originally intend to go when making the promise, then intentions of not fulfilling the promise are considered hypocrisy (nifaq). However, if one did not specify a time and did not resolve not to fulfill it, then it is not hypocrisy. And whoever swears to do something and immediately adds "in sha' Allah" (if God wills), is not required to fulfill their oath and does not break it by not doing it.