ML-DSA add byte array APIs

[krwq]

This also addresses optional feedback from my last COSE PR.

[Copilot]

Pull Request Overview

This PR adds new byte[]-based overloads for ML-DSA export and signing APIs and updates existing code/tests to adopt these simpler overloads, replacing manual buffer management.

• Introduces byte[] ExportMLDsaPublicKey(), ExportMLDsaSecretKey(), ExportMLDsaPrivateSeed(), SignData(byte[], byte[]?) and VerifyData(byte[], byte[], byte[]?) overloads.
• Updates all X509, PFX, COSE, OpenSSL, and common tests to use the new overloads and removes manual buffer length assertions.
• Extends the reference assembly (ref/System.Security.Cryptography.cs) and implementation (MLDsa.cs, MLDsaImplementation.cs, X509Certificate2.cs, MLDsaX509SignatureGenerator.cs) to support these overloads.

Reviewed Changes

Copilot reviewed 19 out of 19 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/libraries/System.Security.Cryptography/tests/X509Certificates/X509Certificate2PemTests.cs	Use new SignData(data) overload instead of manual buffer
src/libraries/System.Security.Cryptography/tests/X509Certificates/PfxTests.cs	Switch to ExportMLDsaPrivateSeed()/ExportMLDsaSecretKey() overloads
src/libraries/System.Security.Cryptography/tests/X509Certificates/ExportTests.cs	Use ExportMLDsaSecretKey() overload
src/libraries/System.Security.Cryptography/tests/X509Certificates/CertificateCreation/PrivateKeyAssociationTests.cs	Use ExportMLDsaPublicKey() and SignData(data) overloads
src/libraries/System.Security.Cryptography/tests/X509Certificates/CertTests.cs	Use ExportMLDsaSecretKey() overload
src/libraries/System.Security.Cryptography/tests/MLDsaOpenSslTests.Unix.cs	Use SignData(data, context) and ExportMLDsa*() overloads
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X509Certificates/X509Certificate2.cs	Simplify public key comparison using new ExportMLDsaPublicKey()
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X509Certificates/MLDsaX509SignatureGenerator.cs	Return SignData(data) directly
src/libraries/System.Security.Cryptography/ref/System.Security.Cryptography.cs	Add and reorder new byte[] overloads for export and signing APIs
src/libraries/System.Security.Cryptography.Cose/tests/TestKeyRing.cs	Add centralized helper for ECDsa/RSA/MLDsa test keys
src/libraries/System.Security.Cryptography.Cose/tests/System.Security.Cryptography.Cose.Tests.csproj	Include new TestKeyRing.cs in test project
src/libraries/System.Security.Cryptography.Cose/tests/CoseTestHelpers.cs	Delegate to TestKeyRing and remove thread-static duplications
src/libraries/Common/tests/.../MLDsa/MLDsaTestsBase.cs	Update tests to use SignData overloads
src/libraries/Common/tests/.../MLDsa/MLDsaTests.cs	Add null-argument validation for new overloads
src/libraries/Common/tests/.../MLDsa/MLDsaTestHelpers.cs	Cover default ReadOnlySpan<byte> in import tests
src/libraries/Common/tests/.../MLDsa/MLDsaImplementationTests.cs	Add tests for null byte[] source imports
src/libraries/Common/src/System/Security/Cryptography/SlhDsaImplementation.cs	Minor formatting around secret-key export
src/libraries/Common/src/System/Security/Cryptography/MLDsaImplementation.cs	Adapt DuplicatePrivateKey to new export APIs
src/libraries/Common/src/System/Security/Cryptography/MLDsa.cs	Implement new byte[] overloads and strengthen span overload checks

Comments suppressed due to low confidence (1)

src/libraries/System.Security.Cryptography.Cose/tests/TestKeyRing.cs:23

• The property references an undeclared backing variable 'field', which will not compile. You should declare a private ECDsa? field for each property (e.g. '_es256') or use auto-properties with initialization.

        internal ECDsa ES256 => field ??= CreateECDsa(_ec256Parameters, true);

[PranavSenthilnathan]

Consider changing the try/finally to using (PinAndClear.Track(rented)) { }.

[krwq]

I'm not certain what the benefit would be from pinning here - if we do it we should probably do it for all algorithms, not just here

[PranavSenthilnathan]

In .NET 10 JIT produces a 0 length null pointer for both ReadOnlySpan<byte>.Empty and default(ReadOnlySpan<byte>) so I don't think both are needed (unless it does something different downlevel).

[krwq]

This is a really cheap test and we we also have M.B.C which should also respect this, not just .NET 10 and higher

[bartonjs]

The "Does not throw" comments feel misplaced to me. Assuming they are commenting the line that they are the end of, the only possible "throw" would be that seed.Length threw ANE.

They should either be somewhere that makes sense, or just deleted.