Chickens are often times not real long lived animals. It does not happen often, but it is not uncommon to find a bird dead in the morning. I think she might have just died, and nothing to do with the heat. You have a lot of shade there, and dry heat is much different than heat + high humidity.
Shade and water and birds should do fine in temperatures over 100 degrees, much better in dry 100 degrees.
Don't lock them up in the coop, and you could just hose the outside of the coop down early evening and that should drop the inside temperature.
Mrs K
