Generate automatic and manual conversation recaps (#40705)

## Why

Users returning to a conversation need a short summary of its recent progress and next steps.

## What changed

- Generate a recap for eligible idle, unfocused conversations and add it to the transcript.
- Add `/recap` to request the same summary on demand, with visible progress and actionable errors.
- Keep recap requests bounded and isolated, reject stale results, and retry only automatic failures.

## Testing

- Cover automatic and manual generation, request eligibility, failure handling, loading-state cleanup, and temporary-thread configuration.

GitOrigin-RevId: 926a88d6996d67a42809ce2c4091b13fca882b52