Does what happened—a married woman being forced to put a strange man's penis in her mouth and him ejaculating into it, while she refused adultery and the man attempted to rape her—constitute zina (unlawful sexual intercourse) that necessitates the hadd (prescribed punishment) for zina? And what is the difference between this act and zina through the vagina or anus?

You have erred and disobeyed your Lord through a forbidden relationship, which the Prophet, peace and blessings be upon him, called metaphorical fornication, even if it is not actual fornication that necessitates a legal punishment (hadd). The expiation for these actions is sincere repentance by abandoning the sin, feeling remorse, and resolving not to return to it, while also concealing it and not publicizing it. Do not tell your husband what happened, for repentance erases what came before it, and one who repents from a sin is like one who has no sin.