Squealing sound when turning steering wheel all the way?

When you turn that sharp and the steering wheel stops the power steering pump puts a larger load on the belt because the pump pressure goes way up. If the belt is worn at all they can squeal under that load. If the belt is quiet the rest of the time and only squeals with the steering at full lock it becomes a choice, you can replace the belt and see if the new one also squeals under that load, or just avoid turning that sharp. If the belt is more than a few years old changing it and the tensioner might be the better option.
https://www.2carpros.com/articles/replace-serpentine-belt

Once Im replacing the belt and tensioner should I replace the idler pulley as well?

Not unless it's noisy.

Okay. Thank you!

You're welcome.