Use cell source code instead of the concatenated one (#12929)

## Summary

fixes: #12880

## Test Plan

Test against the notebook provided in the issue.

crates/ruff_server/src/fix.rs

@@ -123,11 +123,7 @@ pub(crate) fn fix_all(
             fixes.insert(
                 url.clone(),
                 vec![lsp_types::TextEdit {
-                    range: source_range.to_range(
-                        source_kind.source_code(),
-                        &source_index,
-                        encoding,
-                    ),
+                    range: source_range.to_range(&source, &source_index, encoding),
                     new_text: modified[modified_range].to_owned(),
                 }],
             );