ALL of the table service restaurants are air-conditioned.
Some of the counter-service places, however, have open-air dining areas. They are all covered to provide relief from sun and rain, but they can get a little hot in summer.
Some of the hottest that I have tried are FlameTree BBQ in AK, the Lotus Blossom Cafe in Epcot's China, and especially the Cantina de San Angel in Mexico, which has umbrellas at the tables for cover and not much else. But the food is very good at all of these, so it's a trade-off.