Has there ever been an open military attack between nuclear-armed nations such as what happened between India-Pakistan yesterday?

The major example of two nuclear states actually fighting militarily with significant casualties is when the USSR and China had a border conflict in 1969. Several hundred people died in the process and there were significant fears that this could escalate into a larger war. At the time the USSR had a significant nuclear advantage against the Chinese (who had only tested their first bomb in 1964, and has never had more than a few hundred nuclear weapons, whereas the USSR had many thousands by that point and a variety of delivery vehicles). After several bloody incidents the two countries eventually deescalated.

Separately, this is not something that can fall under the rules of this sub (Because it is too recent) but it is just worth noting that there were significant clashes in the Kashmir region between India and Pakistan even after 1998, when both nations tested nuclear weapons. There were also skirmishes prior to 1998, when both countries were generally known to have nuclear capabilities (India tested a "peaceful" nuclear device in 1974, but nobody really thought it was entirely limited to "peaceful" uses; Pakistan was a suspected clandestine nuclear power since the late 1980s). The Kashmir region has been the subject of border clashes since the late 1980s, though there have been times of greater and less intensity.