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.

crates/ui/src/components/icon.rs

@@ -154,6 +154,7 @@ pub enum IconName {
     Copy,
     CountdownTimer,
     Dash,
+    DatabaseZap,
     Delete,
     Disconnected,
     Download,
@@ -322,6 +323,7 @@ impl IconName {
             IconName::Copy => "icons/copy.svg",
             IconName::CountdownTimer => "icons/countdown_timer.svg",
             IconName::Dash => "icons/dash.svg",
+            IconName::DatabaseZap => "icons/database_zap.svg",
             IconName::Delete => "icons/delete.svg",
             IconName::Disconnected => "icons/disconnected.svg",
             IconName::Download => "icons/download.svg",