Clean up `CompositeMLDsaAlgorithm` and add tests

[PranavSenthilnathan]

Implement CompositeMLDsaAlgorithm, fixing any issues with the existing code/documentation. There is already an implementation which should mostly be correct, so that just needs to be updated. Then add tests for it similar to SlhDsaAlgorithmTests and MLDsaAlgorithmTests for both System.Security.Cryptography and Microsoft.Bcl.Cryptography.

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