Is taking a medical certificate for marriage without examination or a visit to the doctor considered forgery, and does it invalidate the marriage contract?

What you did constitutes forgery. Its ruling depends on the obligation to obey the ruler regarding premarital medical examinations. If the situation of the questioner and his fiancée does not fall under cases where obedience is obligatory, and conducting the examination would be difficult for them, then there is no objection to obtaining the certificates without a real examination. However, if it is easy to do so, then conducting the examination becomes necessary. In any case, the marriage is valid.