Fix bugs with multicursor completions (#28586)

Release Notes:

- Fixed completions with multiple cursors leaving duplicated prefixes.
- Fixed crash when accepting a completion in a multibuffer with multiple
cursors.
- Vim: improved `single-repeat` after accepting a completion, now
pressing `.` to replay the completion will re-insert the completion text
at the cursor position.

crates/multi_buffer/src/multi_buffer.rs

@@ -7694,7 +7694,12 @@ impl ToOffset for Point {
 impl ToOffset for usize {
     #[track_caller]
     fn to_offset<'a>(&self, snapshot: &MultiBufferSnapshot) -> usize {
-        assert!(*self <= snapshot.len(), "offset is out of range");
+        assert!(
+            *self <= snapshot.len(),
+            "offset {} is greater than the snapshot.len() {}",
+            *self,
+            snapshot.len(),
+        );
         *self
     }
 }