What should a husband do after his wife confesses to committing adultery with another person before their engagement, given his prior knowledge of the relationship before marriage and his insistence on asking?

You made a mistake by asking your wife about her bad past, as she is not obliged to inform you, and she made a mistake by informing you. The Sharia commands anyone who has committed a sin to conceal it and to repent, as repentance erases what came before it. Therefore, if your wife is righteous now, do not pay attention to her past or ask her about it. You must conceal her faults so that Allah may conceal yours on the Day of Judgment.