Reduce first session event latency

Select concrete source-generated event metadata from the existing discriminator for both persisted history and live JSON-RPC binding instead of configuring every polymorphic event type.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot-Session: efd4609c-cefc-43ee-933f-7fa85fd68fa5

Author: davkean

dotnet/src/Client.cs

@@ -2828,6 +2828,8 @@ private static JsonSerializerOptions CreateSerializerOptions()
             DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull
         };
 
+        options.Converters.Add(SessionEventJsonConverter.Instance);
+        options.TypeInfoResolverChain.Add(SessionEventJsonTypeInfoResolver.Instance);
         options.TypeInfoResolverChain.Add(ClientJsonContext.Default);
         options.TypeInfoResolverChain.Add(TypesJsonContext.Default);
         options.TypeInfoResolverChain.Add(CopilotSession.SessionJsonContext.Default);