What is the ruling on marriage contracts that were concluded with a just witness and the girl's guardian, based on the assumption that the girl's guardian is the second witness, and that the "just witness" mentioned in the noble Hadith is for confirming that both parties are free from marriage impediments such as fosterage?

The majority of scholars hold that the validity of a marriage requires two just witnesses, and they agree that the guardian cannot be a witness. So, if the guardian witnesses the marriage of his ward along with another witness, the marriage is not valid due to the lack of the stipulated number of witnesses. However, if a number of people attend the عقد (contract) or the marriage is announced thereafter, then the marriage is valid. Witnessing a marriage aims to document it, publicize it, and establish lineage, not merely to ascertain that there are no impediments for the couple.