Expose Composite ML-DSA CNG identifiers

[PranavSenthilnathan]

Fixes #130052

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Pull request overview

This PR exposes Composite ML-DSA identifiers for Windows CNG by promoting existing internal constants to public CngAlgorithm / CngAlgorithmGroup properties, marked as experimental to align with the PQC API posture.

Changes:

• Make CngAlgorithm.CompositeMLDsa and CngAlgorithmGroup.CompositeMLDsa public and annotate them with Experimental(SYSLIB5006).
• Update the System.Security.Cryptography reference assembly to include the new public properties.
• Remove the test-only CngAlgorithm extension shim now that the identifiers are available publicly.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/CngAlgorithm.cs	Exposes CompositeMLDsa as a public experimental algorithm identifier.
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/CngAlgorithmGroup.cs	Exposes CompositeMLDsa as a public experimental algorithm-group identifier.
src/libraries/System.Security.Cryptography/ref/System.Security.Cryptography.cs	Adds the new public properties to the ref contract with Experimental("SYSLIB5006").
src/libraries/Common/tests/System/Security/Cryptography/AlgorithmImplementations/CompositeMLDsa/CompositeMLDsaTestHelpers.Cng.cs	Removes the internal test-only extension helper; tests now use the public API.