And my answer would be 'yes I am aware, thank you',
Well you obviously aren't aware, or you don't care about your dog then. As if you are out in the hottest part of the day (on a hot summer day) walking your dog on the pavements with no shade or water, and your dog is clearly suffering because of it, then by saying you are aware of it, you are actually saying, "yes I'm aware of that, thank you, but I don't actual care that my dog is suffering in this heat so you may as well keep your opinion to yourself"
In any case, if that was your passive aggressive reply, I wouldn't pursue it any further "just to make a point", because it would be obvious that you were a pompous selfish twat and you don't actually care about your dogs wellbeing.. nothing good can come from continuing the conversation with such a person.
But if I passed someone with their dog clearly in distress, I'd ALWAYS ask if they were okay and if they knew about heatstroke, you only have a limited window with heatstroke to take action and I'm betting most the people out walking their dogs in the blazing heat would have no clue what to even do if it happened to them, some of them don't even carry water.