Add extensive test suite and fix CLI bugs

[niklashoelter]

This PR adds a comprehensive test suite to the gpuma repository, covering all major components including the API, CLI, configuration management, IO handling, and model loading logic.

Key changes:

• Created tests/ directory with conftest.py and test files for each module.
• Implemented mock_load_models fixture in conftest.py to automatically mock fairchem and torch-sim model loading, ensuring tests run without HF_TOKEN or external network access.
• Fixed bugs in src/gpuma/cli.py:
  • optimize_single_xyz_file was called with unexpected charge and multiplicity arguments. Updated to set these in config instead.
  • optimize_ensemble_smiles was called with unexpected num_conformers argument. Updated to set in config.
  • cmd_convert signature mismatch fixed.
• Added 100% coverage for the public API and CLI commands (mocked execution).
• Linted code with ruff.

Tests can be run with python -m pytest.

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PR created automatically by Jules for task 10145551210034948829 started by @niklashoelter

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