Fix AttributeError in SYSBackendMethod by separating class identifier from property_ref

[Copilot]

The SYSBackendMethod handler was reassigning bm_class_id from the class identifier to property_ref, then using the same variable to both lookup methods in class_properties and retrieve instances from _class_mapper. This causes AttributeError because ClassMapper._map() attaches instances using class identifiers (e.g., CertCA), not property_ref values (e.g., Cert).

Changes

• Introduced backend_class_id to retain the original class identifier for instance retrieval
• Introduced class_properties_id to hold the property_ref for method lookup in class_properties

# Before
bm_class_id, bm_method = next(iter(bm_root.items()))
try:
    bm_class_id = class_references[bm_class_id]['property_ref']  # Loses original identifier
except KeyError as e:
    ...
getattr(getattr(self._class_mapper, bm_class_id), bm_method)()  # AttributeError

# After
bm_class_id, bm_method = next(iter(bm_root.items()))
backend_class_id = bm_class_id
class_properties_id = bm_class_id
try:
    class_properties_id = class_references[bm_class_id]['property_ref']
except KeyError as e:
    ...
getattr(getattr(self._class_mapper, backend_class_id), bm_method)()  # Correct

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