feat(app-server): turn/steer API

[owenlin0]

This PR adds a dedicated turn/steer API for appending user input to an in-flight turn.

Motivation

Currently, steering in the app is implemented by just calling turn/start while a turn is running. This has some really weird quirks:

• Client gets back a new turn.id, even though streamed events/approvals remained tied to the original active turn ID.
• All the various turn-level override params on turn/start do not apply to the "steer", and would only apply to the next real turn.
• There can also be a race condition where the client thinks the turn is active but the server has already completed it, so there might be bugs if the client has baked in some client-specific behavior thinking it's a steer when in fact the server kicked off a new turn. This is particularly possible when running a client against a remote app-server.

Having a dedicated turn/steer API eliminates all those quirks.

turn/steer behavior:

• Requires an active turn on threadId. Returns a JSON-RPC error if there is no active turn.
• If expectedTurnId is provided, it must match the active turn (more useful when connecting to a remote app-server).
• Does not emit turn/started.
• Does not accept turn overrides (cwd, model, sandbox, etc.) or outputSchema to accurately reflect that these are not applied when steering.