Is it obligatory to repeat the Dhuhr prayer if a traveler intended to perform a complete prayer, and then it became clear that the Imam was praying shortened and combined?

Your prayer is valid, and you are not required to repeat it, because the intention to shorten the prayer (qasr) is not obligatory for the traveler. The original rule for the traveler's prayer is two rak'ahs, and shortening it does not require an intention. Even if the traveler intends to pray the full number of rak'ahs, the Sunnah for him is two rak'ahs.