Box vectors are are not consistently converted after being set

[mattwthompson]

Description
The docs say that box vectors are stored internally with nanometers:

Positions [...] are represented in nanometers [...] internally and when reported.

but the setter will accept (and not convert) units like Angstroms.

Note that this is just a problem of being stored in nanometers vs. compatible units - this is not a problem at export time

Reproduction

When using a setter (with compatible units), the internal state isn't converted to use nanometers:

>>> from openff.toolkit import *
>>> out = ForceField("2.3.0.offxml").create_interchange(Molecule.from_smiles("CCO").to_topology())
>>> out.box = Quantity([1, 2, 3], 'picometer')
>>> out.box
<Quantity([[1. 0. 0.]
 [0. 2. 0.]
 [0. 0. 3.]], 'picometer')>

This isn't a huge deal with nanometers vs. Angstroms since we take care to ensure the correct units at export time. But consistent internal state is desirable.

I think this comes from a missed case here:

>>> from openff.interchange._annotations import _duck_to_nanometer
>>> _duck_to_nanometer([1, 2])
<Quantity([1 2], 'nanometer')>
>>> _duck_to_nanometer(Quantity([0.5, 1.5], "angstrom"))
<Quantity([0.5 1.5], 'angstrom')>

Software versions

• Which operating system and version are you using? macOS 26.3.1
• How did you install Interchange? Pixi, latest development branch
• What is the output of running conda list?