Is it obligatory to break the fast on a Ramadan day if you woke up shortly before Fajr and found a brownish discharge indicating the onset of menstruation, so you did not form the intention to fast, and then nothing came down after Fajr?

If you doubt whether purity was attained before or after dawn, then it is ruled that it was attained after dawn, and you are not required to abstain [from food and drink], and you are permitted to break your fast. However, if you are certain of purity before dawn, then you must fast that day. If the intention to fast overnight was missed, then you are required to abstain and make up that day, because the intention to fast overnight is a condition for the validity of the fast, and whoever does not intend to fast, their fast is not valid.