Is the marriage contract considered valid after repentance, knowing that there was a previous relationship (adultery) between the two parties before the marriage?

You did well to repent, and it was obligatory to stop the messages. A condition for marrying a woman from the People of the Book is that she must be chaste (not an adulteress), based on the Almighty’s saying: "And chaste women from among the believers and chaste women from among those who were given the Scripture." If the woman has not repented from adultery, your marriage to her is invalid and must be annulled. It is better for a Muslim to marry a righteous Muslim woman.