Is expiation (kaffarah) obligatory for someone who swore not to masturbate for a month, then involuntarily emitted semen due to fantasies, without intending to ejaculate or masturbate?

What you did, forms of masturbation, is forbidden by Islamic law. You are required to offer expiation for breaking your oath, unless you specifically intended not to masturbate with your hand. In that case, expiation is not required, but you must make sincere repentance and rid yourself of this habit. Your statement "I did not intentionally ejaculate" contradicts the oath. If you did not intend to take the oath, it would be a vain oath (yamin laghw) for which no expiation is required.