Is a person held accountable for a wet dream if the seminal fluid was not fully discharged, and this occurred at the moment of waking up?

There is no sin upon you in what you mentioned, so long as you did not expel the rest of the semen by masturbation. If you did that, you would be sinful; otherwise, there is no sin upon you. And if some semen was discharged, then ghusl (ritual bath) is obligatory upon you.