I’ve always assumed it’s not so much that they fail safe, but there’s a lot of engineering worked into them to make sure they work very reliably. I assume the spring holds the breaker closed, not pushes it open. If the spring breaks or weakens, the circuit opens. I know they have to have a way to deal with arcing so the contacts don’t weld closed. They have two ways of tripping, over heating and over amperage, not so much for redundancy, but to protect against different failure modes the circuit may experience, but it’s still good that if you go beyond the rated amperage for some reason, the heat will eventually trip the breaker (hopefully) or vice versa.
Might be stupid, but it’s not an unheard of diagnosing method for finding a short. I’ve heard multiple people (often factory workers) comment that the fastest way to find a short it to replace the fuse with a copper lug and look for the smoke.