InvenTree has SSTI in PART_NAME_FORMAT bypasses CVE-2026-27629 fix via {% if part.pk %} sandbox escape

InvenTree is an Open Source Inventory Management System. From 1.2.3 to 1.2.6, the fix for CVE-2026-27629 upgraded the PART_NAME_FORMAT validator to use jinja2.sandbox.SandboxedEnvironment. However, the actual renderer in part/helpers.py was not updated and still uses the non-sandboxed jinja2.Environment. Additionally, the validator uses a dummy Part instance with pk=None, which allows conditional template expressions to behave differently during validation versus production rendering. A staff user with settings access can craft a template that passes validation but executes arbitrary code during rendering. This issue requires access by a user with granted staff permissions. This vulnerability is fixed in 1.2.7 and 1.3.0.