Rename SecretKey to PrivateKey for ML-DSA

[bartonjs]

...SecretKey... got renamed to ...PrivateKey... during the API review for ML-DSA.

Similar to the cleanup for SLH-DSA, this does not change anything in the Interop classes or the crypto shim, but should have everything else.

• All remaining usages of "secret key" found by VS in System.Security.Cryptography are related to HMAC, AES-GCM, or AES-CCM.
• All remaining usages of "secretkey" found by VS in System.Security.Cryptography are for Composite ML-DSA, one place in SLH-DSA that got missed, or the SecretKeySeed/SecretKeyPrf in SLH-DSA tests.
• Comments above do not include src/Common/src/Interop

I did a manual check of the approved shape against the ref.cs, and it matches except for SignMu/VerifyMu (which were added in a followup review).

Fixes #117911.

[Copilot]

Pull Request Overview

This PR renames "SecretKey" to "PrivateKey" throughout the ML-DSA (Module-Lattice-Based Digital Signature Algorithm) implementation to align with API review decisions. The change is primarily cosmetic, updating method names, property names, variables, and resource strings while maintaining the same underlying functionality.

Key changes include:

• Renaming methods from ExportMLDsaSecretKey to ExportMLDsaPrivateKey and ImportMLDsaSecretKey to ImportMLDsaPrivateKey
• Updating the SecretKeySizeInBytes property to PrivateKeySizeInBytes in MLDsaAlgorithm
• Changing internal field names and variables from secretKey/hasSecretKey to privateKey/hasPrivateKey

Reviewed Changes

Copilot reviewed 30 out of 30 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
ref/System.Security.Cryptography.cs	Updates public API surface with renamed methods and properties
src/System/Security/Cryptography/MLDsa*.cs	Core implementation files with renamed methods and internal state
tests/**/*.cs	Test files updated to use new method names and variable names
src/Resources/Strings.resx	Resource strings updated from "secret key" to "private key"
Common/src/**/*.cs	Shared implementation code with renamed methods and variables

Comments suppressed due to low confidence (1)

src/libraries/System.Security.Cryptography/ref/System.Security.Cryptography.cs:1850

• The error message resource string 'Argument_PrivateKeyWrongSizeForAlgorithm' is referenced but doesn't exist in the updated resource files. The resource files show 'Argument_SecretKeyWrongSizeForAlgorithm' was removed but no corresponding 'Argument_PrivateKeyWrongSizeForAlgorithm' was added.

    {

[PranavSenthilnathan]

Looks like this was missed in the COSE tests: https://github.com/dotnet/runtime/blob/main/src/libraries/System.Security.Cryptography.Cose/tests/TestKeyRing.cs#L94

[bartonjs]

There's always just one more library to check... 😄

[dotnet-policy-service]

Added needs-breaking-change-doc-created label because this PR has the breaking-change label.

When you commit this breaking change:

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Tagging @dotnet/compat for awareness of the breaking change.

[bartonjs]

Doc template filled out: dotnet/docs#47691