Is it considered a state of ritual impurity (janabah) that necessitates a full ritual bath (ghusl) if there is an unknown spot after sleeping, with doubt about having a wet dream, knowing that the person only washed the visible impurity and missed the Fajr prayer?

If you doubt whether what has exited you is maniy (seminal fluid) or urine, you may consider it urine and purify whatever it has touched of your body and clothes, then perform wudu' and pray. If you intentionally missed the Fajr prayer, you must repent to Allah and make up the prayer.