Release Notes:
- Adds UI affordances to the assistant panel to show which messages have
been cached
- Migrate cache invalidation to be based on `has_edits_since_in_range`
to be smarter and more selective about when to invalidate the cache and
when to fetch.
<img width="310" alt="Screenshot 2024-08-16 at 11 19 23 PM"
src="https://github.com/user-attachments/assets/4ee2d111-2f55-4b0e-b944-50c4f78afc42">
<img width="580" alt="Screenshot 2024-08-18 at 10 05 16 PM"
src="https://github.com/user-attachments/assets/17630a60-7b78-421c-ae39-425246638a12">
I had originally added the lightening bolt on every message and only
added the tooltip warning about editing prior messages on the first
anchor, but thought it looked too busy, so I settled on just annotating
the last anchor.
