return promptRequired when no turn is running

[lifeizhou-ap]

Summary

A steering request can arrive just as the active turn ends. Codex ACP currently starts a new turn itself and returns startedNewTurn, leaving the client without a normal session/prompt request to await or cancel.

This PR adds the opt-in behavior introduced in Claude ACP #919.

When the client sends:

{
  "_meta": {
    "steering": {
      "idleBehavior": "promptRequired"
    }
  }
}

Codex ACP:

• Returns injected if a turn is running.
• Returns promptRequired if the turn has ended, without starting another turn.
• Allows the client to submit the message through session/prompt.
• Preserves startedNewTurn when the option is not provided.

Verification

• npm run typecheck
• npm test
• npm run build
• npm run example:steering