Is delaying ritual bathing (ghusl) and ablution (wudu') until sunrise, due to obsessive thoughts (waswasa) regarding purity, considered an intentional abandonment of the Fajr prayer? And is it correct to say that whoever intentionally abandons a single prayer has committed an act of disbelief (kufr)?

The cure for obsessive thoughts (waswas) is to disregard them and pay them no attention. There is a difference of opinion regarding intentionally abandoning prayer. What you did, if you were able to repel these obsessive thoughts but failed to do so, is a grave error for which sincere repentance is due. However, if you were unable to repel them, then there is no sin upon you, Allah willing. You must strive against yourself to rid yourself of these obsessive thoughts, completely ignore them, and not yield to them.