CoreSync.Http.Client fails under NativeAOT: reflection-based System.Text.Json and MessagePack Typeless/contractless resolvers #22

We sync a .NET 11 iOS app (NativeAOT, PublishAot=true) through CoreSync.Http.Client 0.1.129 + CoreSync.Sqlite 0.1.129. Under CoreCLR it works; under NativeAOT the first sync fails after the local GetChanges with SynchronizationException wrapping NotSupportedException:

'System.Text.Json.Serialization.Converters.DictionaryOfTKeyTValueConverter`3[
  System.Collections.Generic.Dictionary`2[System.String,System.Int32],System.String,System.Int32]'
is missing native code or metadata.

Cause: SyncProviderHttpClient uses PostAsJsonAsync / ReadFromJsonAsync<T> without a JsonTypeInfo, so System.Text.Json builds converters by reflection (BulkSyncChangeSet.ChangesByTable is Dictionary<string, int>). NativeAOT has no code for value-type generic instantiations it did not see at compile time.

We worked around it in the app by declaring a source-generated JsonSerializerContext for BulkSyncChangeSet, BulkChangeSetUploadItem, SyncAnchor, SyncVersion and SyncItem[] that is never used, only so the compiler emits those instantiations. With that, a full download (143 rows) succeeded under NativeAOT.

The second risk is CoreSyncMessagePackSerializer.Options = TypelessContractlessStandardResolver.Options: MessagePack's dynamic resolvers generate IL at run time (not available under NativeAOT), and Typeless resolves types by name. Our runs used the JSON path, so we have not hit it yet; we expect bulk upload/download through MessagePack to fail the same way.

Suggestions

1. Accept a JsonSerializerOptions/IJsonTypeInfoResolver in SyncProviderHttpClientOptions (or ship a source-generated CoreSyncJsonContext for the HTTP DTOs) and pass JsonTypeInfo to the *AsJsonAsync calls.
2. Offer an AOT-safe MessagePack configuration (source-generated formatters via MessagePack v3's analyzer/generator, or a non-Typeless contract for SyncItem values), keeping wire compatibility with existing servers and clients.
3. Mark the assemblies IsAotCompatible once the warnings are gone.

Versions: CoreSync.Http.Client 0.1.129, CoreSync.Sqlite 0.1.129, MessagePack 2.5.187, .NET SDK 11.0.100-rc.2, iOS simulator NativeAOT.