fix: change StringEnum back to record

[martincostello]

Resolves #766

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Before the change?

• Change StringEnum<T> back to a sealed record to mitigate the performance regression reported in [BUG]: Performance regression from 3.1.0 #766. My guess would be that readonly struct copy-by-value to happen when values are passed around, where as before they would be passed by reference.

After the change?

• Doesn't fully restore the previous performance, but there's a noticable improvement in Bump Octokit.Webhooks.AspNetCore martincostello/costellobot#2742.
  

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Technically, this is a breaking change from a non-reference type to a reference type (but puts things back as they were before 3.0.0, so you could argue it isn't as 3.1.0 wasn't).

• Yes
• No

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[github-actions]

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[Copilot]

Pull Request Overview

Reverts StringEnum<T> from a readonly struct back to a sealed record to address performance regression issues. The change removes custom equality operators and updates the Equals method to handle nullable parameters properly.

• Changes type declaration from readonly struct to sealed record
• Removes custom equality operators (== and !=) that are now handled by record semantics
• Updates Equals method parameter to be nullable and adds null safety checks

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