multi_buffer: Fix up some anchor checks (#43454)

Release Notes:

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

crates/multi_buffer/src/multi_buffer_tests.rs

@@ -3050,7 +3050,9 @@ async fn test_random_multibuffer(cx: &mut TestAppContext, mut rng: StdRng) {
 
         for _ in 0..10 {
             let end_ix = rng.random_range(0..=text_rope.len());
+            let end_ix = text_rope.floor_char_boundary(end_ix);
             let start_ix = rng.random_range(0..=end_ix);
+            let start_ix = text_rope.floor_char_boundary(start_ix);
             assert_eq!(
                 snapshot
                     .bytes_in_range(MultiBufferOffset(start_ix)..MultiBufferOffset(end_ix))