diagnostics: Keep diagnostic excerpt ranges properly ordered (#42298)

Fixes ZED-2CQ

We were doing the binary search by buffer points, but due to await
points within this function we could end up mixing points of differing
buffer versions.

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/sum_tree/src/cursor.rs

@@ -448,7 +448,7 @@ where
         aggregate: &mut dyn SeekAggregate<'a, T>,
     ) -> bool {
         assert!(
-            target.cmp(&self.position, self.cx) >= Ordering::Equal,
+            target.cmp(&self.position, self.cx).is_ge(),
             "cannot seek backward",
         );