What is the ruling on doubting the emission of maniyy (sperm) or madhiy (pre-seminal fluid) after masturbating through thought, and the obsessive worry about the bed being defiled by them, despite not seeing any trace of them?

You are not required to purify the bed merely out of doubt. Masturbation, whether by hand or otherwise, is forbidden. If masturbation occurs merely through thought, it is not forbidden, but it is better to avoid it. If semen is discharged through masturbation or a wet dream, then ghusl (ritual bath) is obligatory. It is advised to turn away from all obsessive thoughts.