fix(realtime): implement synchronous ReceiveUpdates

[sylvesterkaczmarek]

Summary

• implement both synchronous RealtimeSessionClient.ReceiveUpdates overloads
• reuse the existing WebSocket receive pipeline and typed update deserialization
• avoid synchronization-context capture in the receive path used by sync-over-async
• add regressions for both protocol and typed synchronous enumeration

Fixes #1363.

Validation

• dotnet test tests/OpenAI.Tests.csproj --filter FullyQualifiedName~RealtimeUnitTests --no-restore
• 12/12 passed on .NET 8, .NET 9, and .NET 10
• git diff --check

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-07T07:42:56.499423Z	deb5c39	PR opened
🔒 Security Review	✅ Completed 2026-09-07T07:43:59.384273Z	deb5c39	PR opened

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