Add missing null check for SlhDsa's constructor

[vcsjones]

A type that is derived from SlhDsa could pass null in to the algorithm parameter. This adds a null check and a test to prevent it. This makes SlhDsa consistent with MLKem and MLDsa's constructor checks.

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[Copilot]

Pull Request Overview

This PR adds a null check to the SlhDsa constructor and introduces a corresponding unit test to ensure passing null for the algorithm parameter throws ArgumentNullException.

• Added ArgumentNullException.ThrowIfNull(algorithm) to the SlhDsa constructor for null guarding.
• Created a new [Fact] in SlhDsaContractTests to validate that the constructor throws when given null.

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/SlhDsa.cs	Added null-check in protected constructor
src/libraries/Common/tests/.../SlhDsaContractTests.cs	Added test ArgumentValidation_Ctor_NullAlgorithm

Comments suppressed due to low confidence (2)

src/libraries/Common/tests/System/Security/Cryptography/AlgorithmImplementations/SlhDsa/SlhDsaContractTests.cs:54

• [nitpick] The test method name is slightly inconsistent with the existing naming pattern (NullArgumentValidation). Consider renaming it to NullArgumentValidation_Ctor_NullAlgorithm for consistency with other argument validation tests.

public static void ArgumentValidation_Ctor_NullAlgorithm()

src/libraries/Common/src/System/Security/Cryptography/SlhDsa.cs:58

• Consider updating the XML doc comment for this constructor to include an <exception> tag documenting that passing a null algorithm throws ArgumentNullException.

ArgumentNullException.ThrowIfNull(algorithm);

[vcsjones]

/ba-g Android leg timed out and is tracked in #115955