What is the legal ruling regarding dealing with a pregnant wife who was discovered to have had an illicit relationship (without committing adultery) with a married man before marriage, and she has sworn to repent?

You made a mistake by asking about your wife's past relationships and seeking out their details, and whoever told you about them also made a mistake. As long as your wife has repented and no suspicion has arisen from her, you must conceal her past and live with her kindly. A man must uphold his responsibility of guardianship over his wife, close off avenues of temptation for her, establish Allah's boundaries in his home, cooperate with his wife in obeying Allah, and then have a good opinion of her.