Well, the dealer is not wrong about the cause of the squeak. Brake rotors will build up a surface rust over the course of several hours. This will then produce various kinds of unpleasant (though technically harmless) noises. So there is likely nothing at all wrong with the brakes - in terms of their real function which is about making the car stop.
There are various means of controlling brake squeal. I think that you should go back to the dealer where you bought it and continue to annoy them. Try to find a time when it will be raining or be very humid. Leave the car with them overnight so that someone there can drive it first thing in the morning. This will confirm for them that there is an annoying squeal.