fix(dotnet): handle unknown session event types gracefully

[Ron537]

Summary

When the Copilot CLI introduces new session event types before the SDK is updated,
SessionEvent.FromJson() throws a JsonException (due to FailSerialization policy),
breaking both GetMessagesAsync and real-time event dispatch.

Fixes #880

Changes

• UnknownSessionEvent — concrete fallback type preserving RawType and RawJson
• SessionEvent.TryFromJson — public partial method that returns UnknownSessionEvent instead of throwing, with warning logging
• GetMessagesAsync — uses TryFromJson so unknown events don't fail the entire history
• OnSessionEvent — uses TryFromJson so unknown real-time events don't crash the RPC handler
• 10 unit tests covering known/unknown deserialization, fallback behavior, and mixed event lists

Approach

Follows the Python SDK pattern (SessionEventType.UNKNOWN + _missing_()) — unknown events are handled gracefully by default with no opt-in parameter needed.

No generated code changes required — TryFromJson is added via partial class extension.

[SteveSandersonMS]

Thanks for proposing this, @Ron537!

I was chatting with @stephentoub and, while we definitely agree that we need to add forward-compatibility with unknown event types, we suspect a preferred solution is to use IgnoreUnrecognizedTypeDiscriminators on the base SessionEvent type, and make it non-abstract. This will mean that unknown event types pass through as the base type.

We don't think it's desirable to add a general UnknownSessionEvent type. If we did that, then once the event type does become known in the next SDK update, that creates a different breaking change for any code written to look for UnknownSessionEvent. Plus, exposing the raw JSON could lead people to take dependencies on internal implementation details from the JSON. So if you don't mind, we will prefer not to do that, and just go with an IgnoreUnrecognizedTypeDiscriminators-style solution.

[Ron537]

Thanks @SteveSandersonMS & @stephentoub !

Great suggestion — I've updated the PR to use IgnoreUnrecognizedTypeDiscriminators = true on the [JsonPolymorphic] attribute and made SessionEvent non-abstract with a virtual string Type => "unknown" default.

Changes are now entirely in the codegen (scripts/codegen/csharp.ts) + regenerated output — no runtime code changes needed. The previous UnknownSessionEvent / TryFromJson approach has been removed.