Is the husband's repeated request for a specific type of food considered a solution for an oath sworn by the wife not to prepare it unless he requests it from her, or is expiation for the oath required of her?

The oath is based on the intention of the one who swears it, according to the النبي (ﷺ) who said: "Deeds are only by intentions." As long as your intention with your oath was not to prepare that food except at your husband's request, then you do not break your oath unless you prepare it for him without his request. Since he asked you to prepare food every day, your oath has been dissolved, and there is nothing against you in preparing it for him.