Use STJ's polymorphism support instead of custom converter

[stephentoub]

Also:

• Mark properties as required if they're required in the schema
• Mark reference types as nullable if they're optional in the schema
• Remove the suppression of CS8618, which is resulting in public types having incorrect nullable annotations
• Annotate enums so the NativeAOT-unfriendly non-generic enum converter isn't needed

[Copilot]

Pull request overview

This pull request modernizes the JSON serialization for session events by replacing a custom polymorphic JSON converter with System.Text.Json's built-in polymorphism support. The changes improve NativeAOT compatibility, add proper nullable annotations to reference types, and mark properties as required when they are required by the schema.

Changes:

• Replaced custom SessionEventConverter with [JsonPolymorphic] and [JsonDerivedType] attributes on the base SessionEvent class
• Added nullable annotations (?) to optional reference types and array properties
• Added required modifier to properties that are required in the schema
• Updated enum declarations with [JsonConverter(typeof(JsonStringEnumConverter<T>))] and [JsonStringEnumMemberName] attributes for NativeAOT compatibility
• Updated usage code to use pattern matching instead of string-based type checking
• Removed CS8618 warning suppression

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 3 comments.

File	Description
nodejs/scripts/generate-csharp-session-types.ts	Updated generator to add nullable annotations to optional types, add required modifier to required properties, generate JsonPolymorphic attributes instead of custom converter, and add NativeAOT-friendly enum attributes
dotnet/src/Generated/SessionEvents.cs	Regenerated file reflecting all generator changes - uses JsonPolymorphic attributes, has proper nullable annotations, required modifiers on properties, and NativeAOT-compatible enum declarations
dotnet/src/Session.cs	Updated implementation code to use pattern matching (case AssistantMessageEvent:) instead of string comparisons (evt.Type == "assistant.message") and updated documentation examples
dotnet/src/Client.cs	Removed unused imports (Newtonsoft.Json.Linq, System.Text.Json.Nodes) and simplified serializer options by removing converter loop

Comments suppressed due to low confidence (2)

dotnet/src/Session.cs:206

• The code uses pattern matching without capturing variables (case AssistantMessageEvent:), but then tries to access type-specific properties (evt.Data?.Content). Since evt is still of type SessionEvent, this will not compile. The code should use pattern matching with variable declarations, for example: case AssistantMessageEvent assistantMessage: Console.WriteLine($"Assistant: {assistantMessage.Data?.Content}");

    ///     switch (evt)
    ///     {
    ///         case AssistantMessageEvent:
    ///             Console.WriteLine($"Assistant: {evt.Data?.Content}");
    ///             break;
    ///         case SessionErrorEvent:
    ///             Console.WriteLine($"Error: {evt.Data?.Message}");
    ///             break;

dotnet/src/Session.cs:335

• The code checks if evt is AssistantMessageEvent but then tries to access evt.Data?.Content without casting. Since evt is still of type SessionEvent, which doesn't have a Data property with a Content field, this will not compile. The code should use pattern matching with a variable declaration, for example: if (evt is AssistantMessageEvent assistantMessage) Console.WriteLine($"Assistant: {assistantMessage.Data?.Content}");

    ///     if (evt is AssistantMessageEvent)
    ///     {
    ///         Console.WriteLine($"Assistant: {evt.Data?.Content}");

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 4 changed files in this pull request and generated no new comments.

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[SteveSandersonMS]

Thanks very much @stephentoub!