Will all your team members also think about those edge cases when changing that part of the code? Will they ensure the behavior is the same when a library dependency is updated?

So, tests catch edge cases that someone else thought of but not everyone might have. This "not everyone" includes yourself, either yourself from the future (e.g. because some parts of the product are not so fresh in your mind), or yourself from now (e.g. because you didn't even know there was a requirement that must be met and your change here broke a requirement over there).

To put an easy to understand example, vulnerability checkers are still tests (and so are linters and similar tools, but let's focus on vulnerabilities). Your post implies you don't need them because you can perfectly prevent a vulnerability from ever happening again once you know about it, both because you write code that doesn't have that vulnerability and because you check that your dependencies don't have that vulnerability.

So, think of tests more like assertions or checksums.