Track caller on <usize as ToOffset>::to_offset (#24503)

To get useful logs when reporting bugs involving offsets out of range

Release Notes:

- N/A

crates/gpui/src/arena.rs

@@ -116,6 +116,7 @@ impl<T: ?Sized> ArenaBox<T> {
         }
     }
 
+    #[track_caller]
     fn validate(&self) {
         assert!(
             self.valid.get(),