Change Key on MLDsaCng to GetKey method

[vcsjones]

As we did for MLKemCng, this changes the CngKey Key property to CngKey GetKey() method that returns a duplicated handle.

[Copilot]

Pull Request Overview

This PR changes the Key property in the MLDsaCng class to a GetKey() method that returns a duplicated CNG key handle, following the same pattern previously implemented for MLKemCng. The change improves resource management by ensuring callers receive their own handle copy instead of sharing the internal handle.

Key changes:

• Property-to-method conversion for better handle management
• Addition of internal KeyNoDuplicate property for internal use
• Test coverage for the new GetKey() method behavior

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
MLDsaCng.cs	Updates API documentation and signature from property to method
MLDsaCng.Windows.cs	Implements GetKey() method with handle duplication logic and adds internal KeyNoDuplicate property
Cng.NotSupported.cs	Updates not-supported implementation and improves error message consistency
System.Security.Cryptography.cs	Updates public API reference from property to method
CertificatePal.Windows.PrivateKey.cs	Updates callers to use internal KeyNoDuplicate property
MLDsaCngTests.Windows.cs	Adds test to verify GetKey() returns distinct duplicated handles

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