Close [BS-2791](https://linear.app/affine-design/issue/BS-2791).
### What Changed?
- Add status filed to `CopilotContextDoc` to querying document embedding processing progress.
- Change `ChipState` from `success` to `finished` to better align with backend server status.
- Add `pollContextDocsAndFiles` API for embedding status polling.
### About Polling
- Set the minimum interval to 1 second and the maximum interval to 30 seconds
- Use exponential backoff and increase the interval by 50% after each poll
- Make sure the interval does not exceed the maximum
1546b76337