* refactor(manager/pixi): extract shared pixi.lock update helper
* test(manager/pixi): cover shared pixi.lock helper directly
Add a dedicated lockfile.spec.ts that exercises updatePixiLockfile
directly (gate, package-file write branch, lockfile maintenance,
unchanged, constraint pass-through, and error handling), and slim the
pixi and pep621 call-site specs down to their own behavior: how each
derives the pixi constraint and that they delegate to the shared helper.
Niels Pardon