Add history-aware prewarming for idle threads

[copyberry]

Add history-aware prewarming for idle threads

What changed

• Add CodexThread::prewarm_with_history() to prepare a WebSocket response with existing conversation history and executed-tool metadata using generate: false. The next turn can reuse the prepared response when its prompt extends that history and request settings match.
• Keep startup prewarming and prewarm() on the existing empty-input path, and skip preparation when the cached WebSocket is ready.
• Label prewarm telemetry by input mode and WebSocket continuation metrics with after_prewarm.
• Rename persistent_mode_enabled to persistent_execution_enabled without changing its behavior.

Testing

Extend WebSocket coverage to check history preservation during reconnect prewarming, reuse of the prepared response on the next turn, and omission of previous_response_id for a non-prefix prompt.

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