Fix multibuffer chunk splitting at UTF-8 boundaries (#57641)

Don't panic when a diff transform boundary falls inside a multi-byte
UTF-8 character. Round split points up to the next char boundary and
advance past any transform boundaries skipped by that adjustment.

Closes FR-16

Release Notes:

- Fixed a crash when deleting words near inline diff boundaries
containing multi-byte characters

crates/multi_buffer/src/multi_buffer_tests.rs

@@ -1527,6 +1527,42 @@ async fn test_basic_diff_hunks(cx: &mut TestAppContext) {
     );
 }
 
+#[gpui::test]
+fn test_text_for_range_with_diff_transform_boundary_inside_multibyte_character(cx: &mut App) {
+    let buffer = cx.new(|cx| Buffer::local("タx", cx));
+    let multibuffer = cx.new(|cx| MultiBuffer::singleton(buffer, cx));
+    let mut snapshot = multibuffer.read(cx).snapshot(cx);
+
+    fn ascii_summary_with_byte_len(byte_len: usize) -> MBTextSummary {
+        let text = "x".repeat(byte_len);
+        MBTextSummary::from(TextSummary::from(text.as_str()))
+    }
+
+    // FR-16 shown a diff transform boundary two bytes into the leading 'タ'.
+    // Build that transform tree directly so this test stays focused on chunk iteration.
+    let mut diff_transforms = SumTree::default();
+    diff_transforms.push(
+        DiffTransform::BufferContent {
+            summary: ascii_summary_with_byte_len(2),
+            inserted_hunk_info: None,
+        },
+        (),
+    );
+    diff_transforms.push(
+        DiffTransform::BufferContent {
+            summary: ascii_summary_with_byte_len("タx".len() - 2),
+            inserted_hunk_info: None,
+        },
+        (),
+    );
+    snapshot.diff_transforms = diff_transforms;
+
+    let text = snapshot
+        .text_for_range(MultiBufferOffset(0)..snapshot.len())
+        .collect::<String>();
+    assert_eq!(text, "タx");
+}
+
 #[gpui::test]
 async fn test_repeatedly_expand_a_diff_hunk(cx: &mut TestAppContext) {
     let text = indoc!(