Snape certainly wasn't expecting heartfelt congratulations from Harry. He was pleasantly surprised to receive any congratulations at all. So he smiled as he inclined his head and replied, "Thank you very much; I'm extremely happy." He glanced sideways at Al and shifted his stance slightly in a minute hint that Al might want to attend to this conversation instead of the previous one.
Actually, any conversation that didn't centre around all the embarrassing things the snakes on his robes could be made to do, would be a huge improvement.