Remove retry wrapper when matching on error kind (#14996)
## Summary
We often match on `ErrorKind` to figure out how to handle an error
(e.g., to treat a 404 as "Not found" rather than aborting the program).
Unfortunately, if we retry, we wrap the error in a new kind that
includes the retry count. This PR adds an unwrapping mechanism to ensure
that callers always look at the underlying error.
Closes https://github.com/astral-sh/uv/issues/14941.
Closes https://github.com/astral-sh/uv/issues/14989.
