Does swallowing regurgitated food after the Adhan invalidate the fast, given that the questioner suffers from obsessive-compulsive disorder (waswasah)?

Your fast is valid, because a fast is only invalidated by the certainty of an intentional act that breaks the fast occurring after the dawn (Fajr) prayer, and these conditions are absent in your situation. You are uncertain about swallowing the regurgitated food, and this occurred shortly before the Fajr adhan, and it was not intentional on your part. Swallowing saliva does not invalidate the fast, and similarly, intentionally swallowing regurgitated food (qals) does not invalidate it if it has not reached a point from which it could be expelled. We advise you to stay away from obsessive thoughts (waswas) and not pay attention to them.