Add DOS, featurizer, FlashMD, and ShiftML regression cases

[EricBoittier]

Locks the ecosystem exports the regression harness names but does not yet run: PET-MAD-DOS 1.0, the PET-MAD featurizer 1.0.0, FlashMD pet-omatpes-v2 at 16 fs, and ShiftML3 model 1. Each case is one structure and compares the outputs listed in regtests/README.md to stored .mts files.

The engine now attaches inputs a model reads without declaring them through requested_inputs() (FlashMD momenta), and a case can set "check_consistency": false when an export's declared dtype does not match the tensors it returns (the featurizer). AtomisticModel loads exports from before _model_capabilities_outputs_names was stored, which FlashMD 0.1.11 needs.

This description was drafted with the help of an LLM.

The four new entries in models.lock still have url: null. export-models.py --check, and therefore tox -e regtests, fails until those files are uploaded to metatensor/metatomic-regtests. Pytest itself passes when the .pt files are in regtests/cache/.

Stacked on the harness commit from metatensor#334 (f45e70d). This PR is on Eric's fork so it can be retargeted onto Luthaf:ecosystem-regtest without pushing to that branch.

Contributor (creator of pull-request) checklist

• Tests updated (for new features and bugfixes)?
• Documentation updated (for new features)?
• Issue referenced (for PRs that solve an issue)?

Reviewer checklist

• CHANGELOG updated with public API or any other important changes?

[Luthaf]

I'm working on this right now =) This needs lab-cosmo/flashmd#58 and lab-cosmo/upet#169 first.