Fix nullable annotations on MLKemAlgorithm and add ToString

[vcsjones]

• MLKemAlgorithm can add some annotations to help the compiler understand nullability. Let's do that.
• Let's override ToString - this helps with output and clarity in xunit theories and other places where an instance is expected to stringify clearly (Perhaps a console log or assert)

[Copilot]

Pull Request Overview

This PR enhances the nullability annotations on MLKemAlgorithm.Equals methods and adds a ToString override for clearer string representations, accompanied by a new corresponding test.

• Annotate Equals(MLKemAlgorithm?) and Equals(object?) parameters with [NotNullWhen(true)] for better nullability inference.
• Override ToString() to return the algorithm’s Name.
• Add Algorithms_ToString xUnit theory to verify ToString() matches Name.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/libraries/Common/src/System/Security/Cryptography/MLKemAlgorithm.cs	Add [NotNullWhen(true)] annotations and ToString override
src/libraries/Common/tests/System/Security/Cryptography/MLKemAlgorithmTests.cs	Add test to verify ToString output

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[PranavSenthilnathan]

/ba-g #116647, #116558, and #106319. This PR shouldn't affect ios/tvos/browser.