The single-quoted bash string passed to sed -i prevented FRONT_VERSION from
being interpolated, so the file content was being silently rewritten to
the literal text "${FRONT_VERSION}". Likewise the double-quoted sed for
pyproject.toml would silently do nothing if the [project] section was
absent while still printing a misleading 'updated' success line.
Replace both with python: json.load/dump for package.json (so the JSON
remains well-formed and 2-space-indented), and a regex section-aware
edit for pyproject.toml that exits non-zero with a warning if no
`version = "..."` line is found, so the success message only prints
when something actually changed.
