fix: raise KeyError (not ValueError) for missing custom property

[speckhard]

Summary

Make the two indexing patterns for missing custom properties agree on the exception type:

call	before	after
mol["missing"]	KeyError	KeyError
mol.get_property("missing")	ValueError	KeyError

Both now raise KeyError with the same message (Property 'X' not found).

Why this is a behavior change worth shipping

• KeyError is the Python-native exception type for "key not found in a mapping-like object", and it's already what mol[key] raised. Code that did try: ... except KeyError: worked for mol[key] but silently failed for mol.get_property(key) because the latter slipped through as ValueError.
• The discrepancy was actively locked in by tests (one in test_atom_molecule.py, one in test_database.py) — those are updated.
• The exception message string is unchanged, so anything matching on the message text still works.

Breaking-change scope

Callers who specifically caught ValueError only for get_property will need to switch to KeyError. There is no in-repo caller doing that; the only places asserting ValueError were the two tests this PR updates.

If broader impact is a concern, this can land in 0.3.0 instead of a 0.2.x patch — happy to defer.

Files

• atompack-py/src/molecule.rs — two PyValueError::new_err(...) → PyKeyError::new_err(...). PyKeyError was already imported in src/lib.rs.
• atompack-py/python/atompack/__init__.pyi — Raises section in the user-facing get_property docstring.
• atompack-py/python/atompack/_atompack_rs.pyi — Raises section also added to the internal stub for consistency.
• atompack-py/tests/test_atom_molecule.py — flip 2 assertions; new test_missing_property_raises_keyerror_consistently locks in the symmetry so future drift fails loudly.
• atompack-py/tests/test_database.py — flip 1 assertion.

Test plan

• Targeted tests (touched files) green after maturin develop --release:

$ uv run --extra dev --locked pytest tests/test_atom_molecule.py tests/test_database.py -v
============================== 42 passed in 1.11s ==============================
tests/test_atom_molecule.py::test_molecule_custom_properties PASSED
tests/test_atom_molecule.py::test_missing_property_raises_keyerror_consistently PASSED
tests/test_database.py::test_database_single_item_mutation_is_copy_on_write[none-False] PASSED
... (all 42)

• Full suite green (no regression elsewhere):

$ uv run --extra dev --locked pytest tests/ -q
124 passed, 6 skipped in 2.27s

(124 = previous 123 + the new test_missing_property_raises_keyerror_consistently.)

• Interactive sanity check:

>>> import atompack, numpy as np
>>> mol = atompack.Molecule.from_arrays(
...     np.zeros((1, 3), np.float32), np.ones((1,), np.uint8))
>>> mol.get_property("missing")
Traceback (most recent call last):
  ...
KeyError: "Property 'missing' not found"
>>> mol["missing"]
Traceback (most recent call last):
  ...
KeyError: "Property 'missing' not found"