Fix MLDsaImplementation on Windows to throw a more clear exception

[vcsjones]

Fixes #117107

[Copilot]

Pull Request Overview

This PR addresses issue #117107 by ensuring that when a public key–only MLDsa instance is used to sign data on Windows, a clear and specific exception is thrown.

• Adds a new test, SignData_PublicKeyOnlyThrows, to validate the exception message clarity when signing data with a public key only.
• Updates the SignDataCore method in the Windows implementation to throw CryptographicException with a clearer error message when no secret key is available.

Reviewed Changes

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

File	Description
src/libraries/Common/tests/System/Security/Cryptography/AlgorithmImplementations/MLDsa/MLDsaTestsBase.cs	Adds a new test to verify that invoking SignData with a public key only throws a clear exception.
src/libraries/Common/src/System/Security/Cryptography/MLDsaImplementation.Windows.cs	Updates SignDataCore to perform an early check for the secret key and throw a clear exception message.

Comments suppressed due to low confidence (2)

src/libraries/Common/tests/System/Security/Cryptography/AlgorithmImplementations/MLDsa/MLDsaTestsBase.cs:284

• [nitpick] Consider renaming the test method to something more descriptive such as 'SignData_WithPublicKeyOnly_ThrowsClearException' to better communicate its purpose.

        public void SignData_PublicKeyOnlyThrows(MLDsaKeyInfo info)

src/libraries/Common/tests/System/Security/Cryptography/AlgorithmImplementations/MLDsa/MLDsaTestsBase.cs:291

• [nitpick] Consider asserting the exact expected exception message rather than only verifying that it doesn't contain 'unknown' to further ensure the exception is as clear as intended.

            Assert.DoesNotContain("unknown", ce.Message, StringComparison.OrdinalIgnoreCase);

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-security, @bartonjs, @vcsjones
See info in area-owners.md if you want to be subscribed.