* fix(security): use SandboxedEnvironment for PART_NAME_FORMAT rendering
- Switch jinja2.Environment to jinja2.sandbox.SandboxedEnvironment in
part/helpers.py to prevent SSTI via template tags in PART_NAME_FORMAT.
- Set pk=1 on the dummy Part instance in the validator to ensure
conditional expressions like {% if part.pk %} are properly evaluated
during validation, closing the sandbox bypass vector.
Fixes GHSA-84jh-x777-8pqq
* Style fixes
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Co-authored-by: Paul <morimori-dev@github.com>
Co-authored-by: Oliver Walters <oliver.henry.walters@gmail.com>
