Fix double read panic in nav history (#22754)

This one seems to be triggered when the assistant's
`View<ContextEditor>` is leased during the call into
`NavHistory::for_each_entry`, which then tries to read it again through
the `ItemHandle` interface. Fix it by skipping entries that can't be
read in the history iteration.

Release Notes:

- N/A

crates/assistant/src/assistant_panel.rs

@@ -4272,6 +4272,10 @@ impl Item for ContextEditor {
             None
         }
     }
+
+    fn include_in_nav_history() -> bool {
+        false
+    }
 }
 
 impl SearchableItem for ContextEditor {