[niklashoelter]

Prepared the codebase for packaging and publishing by:

1. Ensuring all tests pass even without heavy dependencies installed, by improving conftest.py mocks (for torch_sim, fairchem, ase, rdkit, morfeus) and fixing test_models_module.py and test_mol_utils.py failures.
2. Fixing a bug in src/gpuma/optimizer.py where the cached calculator loading function _load_calculator_impl was called but not defined.
3. Updating pyproject.toml to use license = "MIT" (SPDX string) instead of the deprecated table format or redundant classifiers, ensuring compatibility with modern setuptools build backend.
4. Enhancing the GitHub Actions CI workflow to include pip install build twine, python -m build, and twine check steps to verify the package is buildable and metadata is correct.
5. Verified complete docstrings coverage.

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PR created automatically by Jules for task 10218344926544556189 started by @niklashoelter

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