Is intentionally delaying Fajr prayer—after hearing the alarm and turning it off with the intention of waking up later—then waking up after its time has expired, considered an act of disbelief (kufr)? And what is the ruling on one who does that?

There is no sin upon one who sleeps through the prayer until its time has passed, as long as they were not negligent. Negligence is defined as a person sleeping after the prayer's time has begun, while expecting to be overcome by sleep and miss the time. However, if a person wakes up before the prayer's time begins and then sleeps, there is no blame upon them; for prayer is not obligatory until its time has commenced.