Simplify logic & add UI affordances to show model cache status (#16395)

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.

assets/icons/database_zap.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-database-zap"><ellipse cx="12" cy="5" rx="9" ry="3"/><path d="M3 5V19A9 3 0 0 0 15 21.84"/><path d="M21 5V8"/><path d="M21 12L18 17H22L19 22"/><path d="M3 12A9 3 0 0 0 14.59 14.87"/></svg>