You don't have many options other then to try and hammer a smaller socket onto the bolt or replace the tensioner. I would not try and fix this but I would replace the bolt or the entire tensioner.
If you are going to replace it then use a set of vice grips on the bolt and that will grab into the metal and you should be able to extract the bolt.
I attached the process for all this below. Let us know if you have other questions.
https://www.2carpros.com/articles/serpentine-belt-tensioner-replacement
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Friday, February 12th, 2021 AT 7:27 PM