Allow idle threads to prewarm and repair WebSocket connections

[copyberry]

Allow idle threads to prewarm and repair WebSocket connections

Why

A previously used WebSocket can close while a thread is idle. Hosts need a way to prepare the connection before the next turn without repeating warmup on a healthy connection.

What changed

• Expose CodexThread::prewarm() using the existing startup warmup and next-turn handoff.
• Reuse healthy connections and pending warmups, retry failed warmups, and repair closed connections. Treat a finished WebSocket reader task as a closed connection.
• Skip scheduling during an active turn or shutdown, and stop warmup admission before shutdown takes the pending task.
• Expose active_turn_environment_selections() so hosts can inspect the environments captured by the active turn, including those still starting or failed, when authorizing steering.

Testing

Add regression coverage for repeated idle prewarm calls, healthy connection reuse with preserved response continuation, reconnection with generate: false, and warmup rejection after shutdown. Update delayed MCP discovery coverage to verify prewarm repairs an eagerly closed socket and the first turn reuses it.

[github-actions]

Thank you for your submission, we really appreciate it. Like many open-source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution. You can sign the CLA by just posting a Pull Request Comment same as the below format.

---

I have read the CLA Document and I hereby sign the CLA

---

_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the CLA Assistant Lite bot.}