What is the ruling on fasting (whether obligatory, make-up, or supererogatory) if another person's saliva enters the mouth of a fasting person, given the difficulty of avoiding this, especially if the person suffers from obsessive-compulsive disorder?

What you are experiencing is not a residual whisper of doubt, but a significant one. The idea of the saliva of the person being spoken to reaching one's mouth is akin to delusion, and this is from the embellishment and cunning of Satan. There is no cure except to turn away from these whispers.