I am confused. If error-free Imams – M’asumin – know what will occur in the future, why was Imam Reza (A.S) prepared to knowingly drink poison? Would this not be suicide?

The M’asumin may only know things by Allah Almighty’s permission. When He decides to end a life, nothing can alter what He has willed. That includes an Imam knowing that he is about to drink poison.