refactor(fetcher): minor refactoring

[jvoisin]

• Replace a strings.HasPrefix + strings.Cut with a call to strings.CutPrefix.
• Instead of enumerating all possible redirection code, check if the code is 300-something.
• Extract an error constructor outside of its current scope instead of duplicating it 5 times
• Use constant strings for LocalizedError's return values where possible.

[Copilot]

Pull Request Overview

This PR applies several minor refactoring improvements to the response handler fetcher code to make it more readable and maintainable.

• Modernizes string processing by using strings.CutPrefix and strings.SplitSeq
• Simplifies HTTP redirect detection with status code range checking
• Eliminates code duplication by extracting error construction logic

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