In `@newton/_src/utils/import_mjcf.py`:
- Around line 58-107: The cycle detection in _load_and_expand_mjcf currently
uses a global included_files set and falsely flags legitimate repeated includes
as cycles; change this to use a stack-based "active include path" for cycle
detection by replacing or augmenting the included_files param with an
active_stack (e.g., list[str]) that you push the resolved file path onto before
the recursive call and pop it after returning; check membership in this
active_stack to detect true cycles, and do not treat files already processed
elsewhere on the tree (but not on the current stack) as cycles—optionally keep a
separate processed_files set to skip re-parsing but do not use it for cycle
detection. Ensure this logic is applied only for resolved file paths (when not
is_xml_content(resolved)) and reference the function name _load_and_expand_mjcf,
the variables included_files/resolved/active_stack, and the recursive call to
_load_and_expand_mjcf.

In `@newton/_src/utils/import_utils.py`:
- Around line 169-182: The is_xml_content function is too permissive (returns
True for strings containing ? or *), so update is_xml_content to only detect
XML-like content by checking for a leading '<' after stripping whitespace or for
common XML prefixes (e.g. '<?xml') and/or simple tag patterns (e.g. '<tag'
followed later by '>') instead of scanning for characters like '?' or '*' that
appear in valid POSIX/URL paths; modify the logic in is_xml_content to trim the
input and then test for these stricter indicators (use the function name
is_xml_content and its docstring to guide the change).

In `@newton/tests/test_import_mjcf.py`:
- Around line 3690-3744: Tests define local resolver functions (custom_resolver,
tracking_resolver, tracking_resolver in
test_xml_string_input_with_custom_resolver) that accept parameters they don't
use, triggering Ruff ARG001; update those function signatures to silence the
lint by prefixing unused parameters with an underscore (e.g., base_dir ->
_base_dir, include_file -> _include_file) or add a local noqa comment (# noqa:
ARG001) to the function line so the resolver behavior remains unchanged while
removing the lint warning.

In `@newton/_src/utils/import_mjcf.py`:
- Around line 81-87: The XML parsing in import_mjcf.py (and similarly in
import_urdf.py) is vulnerable because the code calls ET.fromstring(...) and
ET.parse(...) on untrusted input (see use in the is_xml_content branch and the
file-path branch); replace these uses with a hardened parser from defusedxml
(import defusedxml.ElementTree at module level, e.g., "from defusedxml import
ElementTree as defused_ET") and call
defused_ET.fromstring(sanitize_xml_content(source)) and
defused_ET.parse(source).getroot() instead of ET.fromstring/ET.parse, update the
module-level imports in both import_mjcf.py and import_urdf.py, and add
defusedxml to the project dependencies.