Seal generated session event types

[stephentoub]

The session event generator currently emits most generated C# DTOs as inheritable classes even when they are leaf types. Sealing those generated types better communicates the intended object model and avoids unnecessary inheritance surface area.

Summary

• Emits sealed classes for session event leaf events, event data DTOs, nested DTOs, JSON-union containers, polymorphic union leaf variants, and the source-generated JSON context.
• Keeps polymorphic bases unsealed because they are required for generated derived types and System.Text.Json polymorphic deserialization.
• Regenerates dotnet/src/Generated/SessionEvents.cs from the updated generator.

Validation

• dotnet build src\GitHub.Copilot.SDK.csproj --no-restore
• dotnet test test\GitHub.Copilot.SDK.Test.csproj --no-build --filter FullyQualifiedName~GitHub.Copilot.SDK.Test.Unit

Generated by Copilot.

[github-actions]

Cross-SDK Consistency Review ✅

This PR modifies only .NET-specific files:

• scripts/codegen/csharp.ts — C# code generator
• dotnet/src/Generated/SessionEvents.cs — regenerated .NET output

The change adds sealed to generated leaf C# classes, which is a C#-specific language feature and best practice. There is no equivalent concept that needs to be mirrored in other SDKs:

• Go: Structs cannot be inherited at all — effectively already "sealed" by the language
• TypeScript/Node.js: No native sealed class concept
• Python: No native sealed class concept

No cross-SDK consistency issues identified. The change is appropriately scoped to the .NET SDK.

Generated by SDK Consistency Review Agent for issue #1330 · ● 103.9K · ◷

[Copilot]

Pull request overview

This PR updates the C# session-event type generator to emit sealed for generated leaf DTOs (events, event payload/data types, nested object DTOs, JSON-union containers, and discriminated-union leaf variants) and regenerates the SessionEvents.cs output accordingly. This reduces unnecessary inheritance surface area while keeping polymorphic base types unsealed for System.Text.Json polymorphic deserialization.

Changes:

• Added DiscriminatedUnionGenerationOptions plumbing to allow sealing discriminated-union leaf variants during generation.
• Updated the generator to emit sealed for nested DTOs, event payload DTOs, event leaf types, and the source-generated JsonSerializerContext.
• Regenerated dotnet/src/Generated/SessionEvents.cs to reflect the new sealing behavior.

Show a summary per file

File	Description
scripts/codegen/csharp.ts	Adds an option to seal discriminated-union leaf classes and applies sealed to generated leaf DTO classes and the JSON context.
dotnet/src/Generated/SessionEvents.cs	Regenerated output marking applicable session-event DTO types as sealed.

Copilot's findings

• Files reviewed: 1/2 changed files
• Comments generated: 0