[Connor Peet (connor4312)]

Summary

Remove the User-only restriction from chat/turnStarted and chat/pendingMessageSet so clients can submit the non-user origins already represented by Message.origin.

• Regenerate all protocol outputs, including affected schema descriptions and Rust, Go, and .NET action documentation.
• Add shared reducer fixtures covering agent, tool, and system-notification input, full message payload preservation, completed history without fabricated response parts, and queued/steering upserts and turn consumption.
• Add the changelog fragment and document the .NET container toolchain used for validation.

This intentionally stays scoped to removing the existing restriction. It introduces no new capability negotiation, authorization mechanism, or provider-delivery contract; human drafts retain their User-only restriction. The existing reducers already preserve non-user messages and need no implementation changes.

Fixes #542.

Validation

• npm run generate
• npm run test — 614 passing; typecheck, lint, metadata/changelog/fragment/generated verification and coverage all pass.
• TypeScript client: npm test — 77 passing; npm run build.
• Shared reducer conformance suites pass in Rust, Go, Swift, Kotlin (JDK 17 container), and .NET (SDK 10 container; 345 passing).
• git diff --check

The initial root test run encountered an existing local-only false positive in the redaction-marker test from ignored VitePress dependency-cache files. Validation passed with that cache temporarily moved outside the scanned tree; the original cache was restored afterward.