Does the joking and saying "I agree" with a colleague on the metro, in the presence of a religious scholar, count as a valid marriage contract, knowing that I suffer from obsessive-compulsive disorder and did not intend marriage with these words?

What transpired between you and your colleague—your saying "I agree" and his silence—is not considered a marriage. This is because it is impermissible for a woman to marry herself, and a marriage is not valid without a guardian (wali), based on the Prophet's (peace be upon him) saying: "There is no marriage except with a guardian" and "A woman may not marry off another woman, nor may she marry herself off." The hadith (definition) "Any woman who marries without the permission of her guardian, her marriage is invalid" further confirms this. The view of the Hanafis permitting marriage without a guardian is weak, so how much more so when your colleague said nothing at all.

You should disregard the obsessive thought that what happened was a marriage. Instead, increase your remembrance of Allah and recitation of the Quran, and be certain that what occurred is not considered a marriage and that the man is a stranger to you (non-mahram (definition)).

What happened with your other colleague—his asking "Will you marry me?" and your negative response—is not considered anything significant, and it indicates a lack of modesty due to impermissible mixing. The advice is to avoid mixing with men, as it entails many sins.