Expose UTF-8 support from FromHexString, TryToHexString, and TryToHexStringLower on the Convert class

[tannergooding]

This resolves #114079

Customer Impact

We exposed the FromHexString, ToHexString, and related APIs back in .NET 5 (2020) and have been getting requests to add UTF-8 support essentially as long. However, we didn't really make a more targeted effort to add native UTF-8 support until .NET 8 when we introduced IUtf8SpanParsable, IUtf8SpanFormattable, and ensured the core primitive numeric types all implemented these interfaces.

Exposing these as part of .NET 10 will unblock customers that have been waiting for this functionality and will give them the higher performance and quality they rely on from built-in APIs.

Testing

All the existing UTF-16 tests were updated to also test the UTF-8 paths.

Risk

Low. This is using the existing UTF-16 algorithms and simply updating them to work for byte. In most cases this is 1-to-1, in a couple places it uses the internal where TChar : IUtfChar<TChar> we use for other shared UTF-8/16 code paths.

[Copilot]

Pull Request Overview

This PR adds UTF-8 support to the Convert class's hex string methods, exposing new overloads that work with ReadOnlySpan<byte> and Span<byte> for UTF-8 encoded hex strings. This complements the existing UTF-16 char-based methods and provides better performance for scenarios working with UTF-8 data.

Key changes:

• Added UTF-8 overloads for FromHexString, TryToHexString, and TryToHexStringLower methods
• Enhanced HexConverter with UTF-8 encoding/decoding support and vectorized optimizations
• Updated tests to cover both UTF-16 and UTF-8 code paths comprehensively

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/libraries/System.Runtime/ref/System.Runtime.cs	Added public API surface for new UTF-8 hex string methods
src/libraries/System.Private.CoreLib/src/System/Convert.cs	Implemented UTF-8 hex string conversion methods and refactored existing code
src/libraries/Common/src/System/HexConverter.cs	Added UTF-8 encoding/decoding support with vectorized implementations
src/libraries/System.Private.CoreLib/src/System/ThrowHelper.cs	Added helper methods for hex-specific exceptions
src/libraries/System.Private.CoreLib/src/System/Text/Unicode/Utf16Utility.cs	Fixed vector comparison to use equality operator
src/libraries/System.Runtime/tests/System.Runtime.Extensions.Tests/System/Convert.ToHexString.cs	Added comprehensive UTF-8 test coverage for ToHexString methods
src/libraries/System.Runtime/tests/System.Runtime.Extensions.Tests/System/Convert.FromHexString.cs	Added comprehensive UTF-8 test coverage for FromHexString methods

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[tarekgh]

Added minor question, LGTM otherwise.

[jeffhandley]

This has my support for introducing in .NET 10 RC1

[tannergooding]

This got M2 sign-off offline.