Are the secretions discharged by a girl when she feels aroused by simple foreplay with her husband after the marriage contract, and without penetration, considered to be in the ruling of prohibited masturbation? And does she have to perform ghusl thereafter?

If a correct marriage contract is established between a man and a woman, she becomes his wife, and whatever is permissible between spouses becomes permissible for them. Therefore, what you mentioned is not unlawful and is not considered masturbation. It appears that what was discharged from you was seminal fluid, which necessitates performing a ritual bath (ghusl).