[PTRun][UnitConverter] Use invariant casing for degree unit tokens

[cui fliter (cuishuang)]

Summary of the Pull Request

Fixes culture-sensitive parsing of CELSIUS and FAHRENHEIT in the PowerToys Run Unit Converter.

DegreePrefixer() was using ToLower(CultureInfo.CurrentCulture) for fixed English unit tokens. In tr-TR, uppercase I is lowercased to the dotless ı, causing inputs such as CELSIUS and FAHRENHEIT to miss their expected branches.

The implementation now uses ToLowerInvariant() for both unit tokens.

PR Checklist

• Closes: [PowerToys Run][Unit Converter] Degree unit parsing fails under Turkish culture #50413
• Communication: I've discussed this with core contributors already. If the work hasn't been agreed, this work might be rejected
• Tests: Added/updated and all pass
• Localization: All end-user-facing strings can be localized
• Dev docs: Added/updated
• New binaries: Added on the required places

Detailed Description of the Pull Request / Additional comments

Updated DegreePrefixer() so internal English unit tokens are normalized independently of the user's current culture.

Added PrefixesDegreesWithTurkishCulture, which sets CultureInfo.CurrentCulture to tr-TR and verifies that:

• CELSIUS becomes DegreeCelsius
• FAHRENHEIT becomes DegreeFahrenheit

The test restores the original culture in a finally block.

Validation Steps Performed

• git diff --check passed.
• Verified that Turkish casing produces celsıus and fahrenheıt.
• Added a regression test for tr-TR.
• The PowerToys unit test project could not be executed in this macOS environment because the required Windows/.NET Visual Studio build environment is unavailable.

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Visual evidence: Not needed — The changed files do not indicate a visible UI change. No visual evidence is expected.

Recommendation

Link the issue this PR fixes using a closing keyword such as Closes #123.

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