Add IParsable/ISpanParsable to Rune

[Copilot]

Closes#60231

Adds the new interfaces, implemented explicitly

[Copilot]

Pull request overview

This PR expands System.Text.Rune’s generic parsing support by making it implement IParsable<Rune> and ISpanParsable<Rune> (explicitly), aligning with the existing explicit IUtf8SpanParsable<Rune> pattern. It updates the System.Runtime ref surface accordingly and adds targeted unit tests for the new parsing behaviors.

Changes:

• Add IParsable<Rune> / ISpanParsable<Rune> to Rune and implement the four explicit interface members using DecodeFromUtf16 + full-consumption checks.
• Update System.Runtime ref to include the new interfaces and explicit member declarations on Rune.
• Add new RuneTests coverage for valid/invalid UTF-16 single-rune inputs for both string and span parsing paths.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/libraries/System.Private.CoreLib/src/System/Text/Rune.cs	Adds explicit IParsable<Rune> / ISpanParsable<Rune> implementations backed by DecodeFromUtf16 and full-input validation.
src/libraries/System.Runtime/ref/System.Runtime.cs	Updates the public contract for Rune to include the two new interfaces and explicit members.
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/Text/RuneTests.cs	Adds theory-based tests for string/span parse and try-parse success and failure cases.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.