SMIRNOFFElectrostaticsCollection.charges does not report cached charges

[mattwthompson]

Using 0.4.0beta1, the .charges attribute does not properly route to the cached charges. I think this is due to a misconfiguration of the Pydantic model.

Run part of the README example:

In [1]: from openff.toolkit import ForceField, Molecule
   ...: from openff.units import unit
   ...:
   ...: from openff.interchange import Interchange
   ...:
   ...:
   ...: # Use the OpenFF Toolkit to generate a molecule object from a SMILES pattern
   ...: molecule = Molecule.from_smiles("CCO")
   ...:
   ...: # Generate a conformer to be used as atomic coordinates
   ...: molecule.generate_conformers(n_conformers=1)
   ...:
   ...: # Convert this molecule to a topology
   ...: topology = molecule.to_topology()
   ...:
   ...: # Define periodicity via box vectors
   ...: topology.box_vectors = unit.Quantity([4, 4, 4], unit.nanometer)
   ...:
   ...: # Load OpenFF 2.0.0 "Sage"
   ...: sage = ForceField("openff-2.0.0.offxml")
   ...:
   ...: # Create an Interchange object
   ...: out = Interchange.from_smirnoff(force_field=sage, topology=topology)
/Users/mattthompson/micromamba/envs/new-models/lib/python3.11/site-packages/foyer/forcefield.py:33: UserWarning: Module openff was already imported from None, but /Users/mattthompson/software/openff-interchange is being added to sys.path
  from pkg_resources import iter_entry_points, resource_filename
/Users/mattthompson/micromamba/envs/new-models/lib/python3.11/site-packages/mdtraj/formats/__init__.py:13: DeprecationWarning: 'xdrlib' is deprecated and slated for removal in Python 3.13
  from mdtraj.formats.trr import TRRTrajectoryFile

Look at the public and private attributes which report charges:

In [2]: out['Electrostatics'].charges
Out[2]: {}

In [3]: out['Electrostatics']._get_charges()
Out[3]:
{TopologyKey with atom indices (0,): -0.13610011111111114 <Unit('elementary_charge')>,
 TopologyKey with atom indices (1,): 0.12639988888888887 <Unit('elementary_charge')>,
 TopologyKey with atom indices (2,): -0.5998001111111111 <Unit('elementary_charge')>,
 TopologyKey with atom indices (3,): 0.04236688888888887 <Unit('elementary_charge')>,
 TopologyKey with atom indices (4,): 0.04236688888888887 <Unit('elementary_charge')>,
 TopologyKey with atom indices (5,): 0.04236688888888887 <Unit('elementary_charge')>,
 TopologyKey with atom indices (6,): 0.04319988888888887 <Unit('elementary_charge')>,
 TopologyKey with atom indices (7,): 0.04319988888888887 <Unit('elementary_charge')>,
 TopologyKey with atom indices (8,): 0.3959998888888889 <Unit('elementary_charge')>}

In [4]: out['Electrostatics'].charges
Out[4]: {}