Add dedicated next-turn state API

[etraut-openai]

Summary

Remote app-server clients need a way to update a thread's next-turn defaults without starting a turn. Today those settings are either local to one TUI or piggybacked on turn/start, which means a second TUI attached to the same thread can miss changes like model, plan mode, or fast mode until a later turn.

This adds a dedicated v2 app-server API for next-turn state and a broadcast notification so connected clients can keep their UI in sync.

This also fixes several bugs reported against the /goal feature where the goal continuation prompt was using outdated turn state because the client was not notifying the server when settings change.

Addresses: #20656, #22090

Changes

• Add thread/turnContext/update and thread/turnContext/updated, including generated JSON schema and TypeScript protocol fixtures.
• Apply partial turn-context updates through the existing session settings path, returning and notifying with the full effective ThreadTurnContext.
• Emit the same notification from turn/start when turn-context overrides change stored defaults.
• Mark the new notification as delivery-required for remote and in-process app-server clients.
• Update the TUI to send setting changes through the new API and refresh active or cached thread state when another client changes the turn context.
• Cover partial updates, explicit serviceTier: null, no-op notification suppression, websocket broadcast delivery, and TUI state refresh behavior.