Add opt-in non-reflective object comparer for NativeAOT - #2308

Summary

Add an explicitly opt-in, non-reflective object-key comparer for NativeAOT while leaving the default security behavior unchanged.

The existing ObjectFallbackEqualityComparer.GetHashCode(object) dynamically invokes the virtual generic comparer factory for the key's runtime type. NativeAOT diagnoses that MakeGenericMethod path with IL2060 and IL3050. Preserving arbitrary existing generic overrides requires keeping the legacy behavior available; replacing it automatically on NativeAOT would change an existing contract.

This PR adds:

• A default-off MessagePack.Security.UseNonReflectiveObjectComparer feature switch, annotated on the net9.0 library asset.
• A separate object comparer selected by that switch, allowing the legacy implementation to be eliminated at publish time.
• One protected overload, IEqualityComparer GetHashCollisionResistantEqualityComparer(Type type), for statically written custom comparer dispatch.
• Contract regression tests and a standalone JIT/native executable.

The diff is limited to four files: one production file, the existing unit test file, and the standalone fixture's project and program. No README, workflow, or solution changes are included. The standalone native fixture is run explicitly; the existing CI is unchanged and does not run it automatically.

Related: #1666 (broader trimming compatibility) and #1202 (preserving the legacy extension contract). This PR addresses only the security object-comparer path; it does not claim whole-library NativeAOT compatibility or resolve the broader scope of #1666.

Explicit opt-in and compatibility

<ItemGroup>
  <RuntimeHostConfigurationOption
      Include="MessagePack.Security.UseNonReflectiveObjectComparer"
      Value="true"
      Trim="true" />
</ItemGroup>

This is an application-wide publish-time choice, not a per-instance setting. Native elimination requires the annotated net9.0 library asset and a .NET 9 or later NativeAOT toolchain. Consuming an older asset or setting AppContext only at runtime does not provide that guarantee. Configure the switch before MessagePack initializes; runtime toggling is unsupported.

With the switch off, the existing implementation remains unchanged, including JIT generic overrides and NativeAOT applications that already make the needed native instantiations available.

With the switch on:

• Primitive keys, Guid, DateTime and DateTimeOffset retain their keyed hashes and normalization. Primitive generic overrides still participate, with per-type caching.
• Boxed enums retain keyed hashing of the full underlying integer, not ordinary Enum.GetHashCode.
• Arbitrary custom types are rejected unless the new Type overload explicitly returns a comparer.
• Boxed enum-specific generic overrides also require an explicit branch in that overload. Typed generic calls are unchanged.
• Object equality, both comparer interfaces, the parameterless factory, and Clone remain available.

Custom security subclasses can connect an existing collision-resistant generic comparer through a statically known type:

protected override System.Collections.IEqualityComparer
    GetHashCollisionResistantEqualityComparer(Type type)
{
    return type == typeof(MyKey)
        ? (System.Collections.IEqualityComparer)this.GetHashCollisionResistantEqualityComparer<MyKey>()
        : base.GetHashCollisionResistantEqualityComparer(type);
}

Use the same pattern for an enum-specific comparer. The returned comparer must implement non-generic IEqualityComparer, as the legacy object fallback also requires. User-supplied comparers retain the existing obligation to provide collision resistance and equality consistency.

No collision protection is disabled. Applications must still select MessagePackSecurity.UntrustedData when appropriate. No serializer payload format, library dependency, target framework, release workflow, or tracked version is changed. No warning suppressions, linker roots, or dependency annotations are added.

Validation

The following local results were obtained on Windows using SDK 10.0.401 at commit 56fafcdd02b33dbf43599d4fde9b482b321c1c60. Subsequent changes remove documentation, workflow additions, and solution registration, and shorten code comments; executable code and test project configuration are unchanged. The repository's pinned SDK was not changed.

The opt-in native scan graphs contain zero nodes for ObjectFallbackEqualityComparer. The legacy negative control retains that implementation and its diagnostics, rather than suppressing them.

The native fixture covers all eight enum underlying types, a boxed-only enum with no typed enum factory call, primitive hash parity, equivalent floating-point/date representations, ordinary-hash collision fixtures, custom/enum overrides, cache reuse, clone isolation, unsupported-type rejection, and an exact-wire-format untrusted map round trip. The legacy checks use ordinary typed calls to demonstrate the existing NativeAOT behavior when the native generic instantiations are available; they do not claim support for arbitrary unseen types.

Linux execution and the full solution test suite were not performed locally.

dotnet run --project tests\MessagePack.Security.NativeAotTests -c Release -p:PublishAot=false -p:NonReflectiveSecurity=false -- legacy
dotnet run --project tests\MessagePack.Security.NativeAotTests -c Release -p:PublishAot=false -p:NonReflectiveSecurity=true -- opt-in
dotnet publish tests\MessagePack.Security.NativeAotTests -c Release -r win-x64 -p:NonReflectiveSecurity=true -warnaserror -o bin\security-optin
.\bin\security-optin\MessagePack.Security.NativeAotTests.exe opt-in