field_proxy_forwarder.py imports non-existent cimgraph.data_profile.cimhub_ufls (broken, invisible to CI)

[craigpnnl]

gridappsd-field-bus-lib field_proxy_forwarder.py imports cimgraph.data_profile.cimhub_ufls, a data profile that does NOT exist in any cim-graph release from 0.4.3a6 through 0.5.0a7 (verified via the CIM-Graph GitHub contents API at both tags). The import would raise ImportError at module load, but it is invisible to CI because no test imports field_proxy_forwarder / FieldProxyForwarder. Found 2026-07-17 during the cim-graph version review.

Two things needed:

1. Fix or remove the broken cimhub_ufls import (determine the correct profile, or delete the dead code if FieldProxyForwarder is unused).
2. Add tests covering how the library actually USES cimgraph (the field-bus / FieldProxyForwarder path), so a broken cimgraph import or profile reference fails CI instead of hiding. This is the real gap: the field-bus lib's cimgraph usage is currently untested, which is why this slipped through.

Related: the cim-graph floor bump to >=0.5.0a7 (separate change) and the field-bus dependency cleanup.