X25519DiffieHellman agreement with byte-based public keys

[vcsjones]

Contributes to #128040.

This introduces a DeriveRawSecretAgreement that does key agreement with the other party's key as a sequence of bytes instead of forcing creating the instance of X25519DiffieHellman. This reduces allocations in the typical use case of X25519.

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[Copilot]

Pull request overview

This PR extends X25519DiffieHellman with byte-based public-key key agreement APIs and wires them through the platform implementations (OpenSSL/Apple/Windows/CNG), adding native shim entrypoints where needed, plus expanded contract and functional test coverage.

Changes:

• Add DeriveRawSecretAgreement(byte[] otherPartyPublicKey) and DeriveRawSecretAgreement(ReadOnlySpan<byte> otherPartyPublicKey, Span<byte> destination) along with a new abstract DeriveRawSecretAgreementCore(ReadOnlySpan<byte>, Span<byte>).
• Implement the new core overload across OpenSSL/Apple/Windows/CNG implementations and add new native entrypoints for OpenSSL and Apple to derive directly from raw public-key bytes.
• Add/extend tests validating argument checks, disposal behavior, symmetry, vectors, and zero-shared-secret behavior for the new overloads.

Show a summary per file

File	Description
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X25519DiffieHellman.cs	Adds the new public overloads and new abstract core method plus length validation helper.
src/libraries/System.Security.Cryptography/ref/System.Security.Cryptography.cs	Updates the public ref surface to include the new overloads and abstract core method.
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X25519DiffieHellmanImplementation.OpenSsl.cs	Implements the new core overload and switches fallback to the new raw-public-key interop.
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X25519DiffieHellmanOpenSsl.OpenSsl.cs	Same as above for the public OpenSSL-backed type.
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X25519DiffieHellmanImplementation.Apple.cs	Implements the new core overload and routes to the new Apple bytes-based interop.
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X25519DiffieHellmanImplementation.Windows.cs	Implements the new core overload and refactors shared Windows derivation path.
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X25519DiffieHellmanCng.Windows.cs	Implements the new core overload and factors shared public-key handling.
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X25519DiffieHellmanCng.cs	Declares the new protected override in the partial type.
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X25519DiffieHellmanImplementation.NotSupported.cs	Adds the new core overload stub for unsupported platforms.
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X25519DiffieHellmanOpenSsl.NotSupported.cs	Adds the new core overload stub (currently throws NotImplementedException).
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/Cng.NotSupported.cs	Adds the new core overload stub in the CNG PNSE implementation.
src/libraries/Common/src/Interop/Unix/System.Security.Cryptography.Native/Interop.EvpPkey.X25519.cs	Adds interop for the new OpenSSL native entrypoint deriving from raw public-key bytes.
src/native/libs/System.Security.Cryptography.Native/pal_evp_pkey_x25519.h	Declares CryptoNative_X25519DeriveSecretAgreementWithPublicKey.
src/native/libs/System.Security.Cryptography.Native/pal_evp_pkey_x25519.c	Implements the new OpenSSL helper by importing the peer key and deriving.
src/native/libs/System.Security.Cryptography.Native/entrypoints.c	Exposes the new OpenSSL entrypoint via the resolver table.
src/libraries/Common/src/Interop/OSX/System.Security.Cryptography.Native.Apple/Interop.X25519.cs	Adds interop for the new Apple bytes-based derivation export.
src/native/libs/System.Security.Cryptography.Native.Apple/pal_swiftbindings.swift	Implements AppleCryptoNative_X25519DeriveRawSecretAgreementWithBytes and shares logic via a helper.
src/native/libs/System.Security.Cryptography.Native.Apple/pal_swiftbindings.h	Declares the new Apple export.
src/native/libs/System.Security.Cryptography.Native.Apple/entrypoints.c	Exposes the new Apple export via the resolver table.
src/libraries/System.Security.Cryptography/tests/X25519DiffieHellmanContractTests.cs	Adds contract tests for the new overloads and adds callback plumbing for the new core method.
src/libraries/System.Security.Cryptography/tests/X25519DiffieHellmanBaseTests.cs	Adds functional tests covering new overloads (symmetry, vectors, public-key-only, zero-secret).

Copilot's findings

• Files reviewed: 21/21 changed files
• Comments generated: 3

[Copilot]

Copilot's findings

• Files reviewed: 21/21 changed files
• Comments generated: 1