Utf16.IsValid and Utf8/16.IndexOfInvalidSubsequence

[lilinus]

Fixes #118018
Fixes #118113

[Copilot]

Pull Request Overview

This PR adds new UTF-16 validation APIs to System.Text.Unicode, introducing Utf16.IsValid and Utf16.IndexOfInvalidSubsequence methods alongside similar functionality for UTF-8. It also refactors existing code to use these new standardized validation APIs instead of custom validation logic.

• Adds new Utf16 class with validation methods IsValid and IndexOfInvalidSubsequence
• Adds Utf8.IndexOfInvalidSubsequence method to complement existing Utf8.IsValid
• Refactors multiple codebases to use the new standardized validation APIs

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
System.Private.CoreLib.Shared.projitems	Adds the new Utf16.cs file to the build
Utf16.cs	New class implementing UTF-16 validation methods
Utf8.cs	Adds IndexOfInvalidSubsequence method and updates class documentation
System.Runtime.cs	Adds public API surface for new UTF-16 and UTF-8 validation methods
HttpEncoder.cs	Refactors surrogate validation to use new Utf16.IndexOfInvalidSubsequence
Normalization.Icu.cs	Replaces custom validation logic with Utf16.IsValid
StringSearchValues.cs	Simplifies surrogate validation using Utf16.IsValid
Utf8UtilityTests.ValidateBytes.cs	Adds test coverage for new Utf8.IndexOfInvalidSubsequence method
Utf16UtilityTests.ValidateChars.cs	Adds test coverage for new Utf16 validation methods

[Copilot]

Grammar error: 'methods that validates' should be 'methods that validate'.

Suggested change

	/// Provides static methods that convert chunked data between UTF-8 and UTF-16 encodings, and methods that validates UTF-8 sequences.
	/// Provides static methods that convert chunked data between UTF-8 and UTF-16 encodings, and methods that validate UTF-8 sequences.

[Copilot]

Grammar error: 'methods that validates' should be 'methods that validate'.

Suggested change

	/// Provides static methods that validates UTF-16 strings.
	/// Provides static methods that validate UTF-16 strings.