ML-KEM: Fix-up ASN.1 reading and writing logic

[vcsjones]

This has a couple of improvements to how MLKem interacts with SubjectPublicKeyInfo and PrivateKeyInfo.

• Thrown exceptions are improved, either by throwing the right exception or not including irrelevant inner exceptions.
• The trailing data check will now account for improperly encoded ASN.1
• Improved handling when derived types have an incorrectly implemented TryExportPkcs8PrivateKeyCore

Fixes #115063

[Copilot]

Pull Request Overview

This PR improves the ASN.1 reading and writing logic for MLKem by enhancing exception handling, validating trailing data, and improving the handling of private key exports.

• Improved exception types and messages for ASN.1 issues.
• Added tests for non-ASN.1 inputs and trailing data.
• Updated logic in MLKem key reader and private key export to validate key lengths and buffer handling.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/libraries/Common/tests/System/Security/Cryptography/MLKemTests.cs	Added tests for non-ASN.1 inputs and trailing data.
src/libraries/Common/tests/System/Security/Cryptography/MLKemContractTests.cs	Renamed and added tests for misbehaving bytes-written scenarios in private key export.
src/libraries/Common/src/System/Security/Cryptography/MLKem.cs	Updated ASN.1 trailing data validation and key reader logic and revised buffer handling in the private key export callback.

Comments suppressed due to low confidence (1)

src/libraries/Common/src/System/Security/Cryptography/MLKem.cs:1823

• In ExportPkcs8PrivateKeyCallback, the initial allocation uses CryptoPool but the fallback allocation uses ArrayPool.Shared. For consistency and to avoid potential pool misuse, allocate all buffers using the same pool.

buffer = ArrayPool<byte>.Shared.Rent(size);

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[bartonjs]

ICYMI:

  Discovering: Microsoft.Bcl.Cryptography.Tests (method display = ClassAndMethod, method display options = None)
  Discovered:  Microsoft.Bcl.Cryptography.Tests (found 360 of 481 test cases)
  Starting:    Microsoft.Bcl.Cryptography.Tests (parallel test collections = on [2 threads], stop on fail = off)
    System.Security.Cryptography.Tests.MLKemNotSupportedTests.ImportSubjectPublicKeyInfo_NotSupported [FAIL]
      Assert.Throws() Failure: Exception type was not an exact match
      Expected: typeof(System.PlatformNotSupportedException)
      Actual:   typeof(System.Security.Cryptography.CryptographicException)
      ---- System.Security.Cryptography.CryptographicException : ASN1 corrupted data.
      Stack Trace:
        /_/src/libraries/Common/tests/System/Security/Cryptography/MLKemNotSupportedTests.cs(35,0): at System.Security.Cryptography.Tests.MLKemNotSupportedTests.ImportSubjectPublicKeyInfo_NotSupported()
           at System.RuntimeMethodHandle.InvokeMethod(ObjectHandleOnStack target, Void** arguments, ObjectHandleOnStack sig, BOOL isConstructor, ObjectHandleOnStack result)
           at System.RuntimeMethodHandle.InvokeMethod(ObjectHandleOnStack target, Void** arguments, ObjectHandleOnStack sig, BOOL isConstructor, ObjectHandleOnStack result)
        /_/src/coreclr/System.Private.CoreLib/src/System/RuntimeHandles.cs(1159,0): at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
        /_/src/coreclr/System.Private.CoreLib/src/System/Reflection/MethodBaseInvoker.CoreCLR.cs(36,0): at System.Reflection.MethodBaseInvoker.InterpretedInvoke_Method(Object obj, IntPtr* args)
        /_/src/libraries/System.Private.CoreLib/src/System/Reflection/MethodBaseInvoker.cs(57,0): at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)
        ----- Inner Stack Trace -----
        /_/src/libraries/Common/src/System/Security/Cryptography/MLKem.cs(1764,0): at System.Security.Cryptography.MLKem.ThrowIfTrailingData(ReadOnlySpan`1 data)
        /_/src/libraries/Common/src/System/Security/Cryptography/MLKem.cs(1173,0): at System.Security.Cryptography.MLKem.ImportSubjectPublicKeyInfo(ReadOnlySpan`1 source)
        /_/src/libraries/Common/src/System/Security/Cryptography/MLKem.cs(1205,0): at System.Security.Cryptography.MLKem.ImportSubjectPublicKeyInfo(Byte[] source)
        /_/src/libraries/Common/tests/System/Security/Cryptography/MLKemNotSupportedTests.cs(36,0): at System.Security.Cryptography.Tests.MLKemNotSupportedTests.<>c.<ImportSubjectPublicKeyInfo_NotSupported>b__4_0()
    System.Security.Cryptography.Tests.AesGcmIsSupportedTests.CtorThrowsPNSEIfNotSupported [SKIP]
      Condition(s) not met: "RuntimeSaysIsNotSupported"
  Finished:    Microsoft.Bcl.Cryptography.Tests

[vcsjones]

runtime-ci run is here: https://github.com/vcsjones/runtime-ci/actions/runs/14720877862/job/41314348123

Only commit since that green run was a merge commit.