Is what the suitor did considered zina (fornication/adultery), and what is its ruling?

If what is meant by "engagement" is the legal marriage contract, then what occurred between you is not forbidden but rather goes against what is preferable. However, if the engagement was merely a promise of marriage, then you have committed a grave sin, as you were still considered strangers to each other. If there was penetration of the glans (ḥashafah), it is considered zina (fornication/adultery) that necessitates the prescribed punishment (hadd). If it was less than that, it is zina in a general sense, which does not necessitate the prescribed punishment but does warrant a severe discretionary punishment (taʿzīr balīgh). In all cases, sincere repentance (tawbah ṣādiqah) and abundant good deeds are obligatory. It is not permissible to complete the marriage after zina except after repentance and confirming the absence of pregnancy (istibrāʾ).