sql, stream: raise for accessing derived attributes

Raising `AttributeError` is definitely wasn't correct here, since it
might push the code to assume it's a wrong entity type. Returning some stub value like `None` also could suggest incorrect derived attribute
value, leading to unexpected behaviour. So adding an error, so it would
propagate and code would need to be adjusted not to rely on derived
attributes, if it actually interacts with sql/stream.

`test_unit` was asserting that derived attr will return `None`, though
it was actually raising `AttributeError`.

src/ifcopenshell-python/test/util/test_unit.py

@@ -329,11 +329,9 @@ class TestCalculateUnitScaleOnLinkedFile(test.bootstrap.IFC4):
     def test_run(self):
         # Regression test: IfcSIUnit.Dimensions is a schema-*derived*
         # attribute that isn't computed for SQLite-linked files (used for
-        # Bonsai's "linked project" large-model workflow), so it returns None
-        # there instead of an IfcDimensionalExponents entity. calculate_unit_scale()
-        # used to access unit.Dimensions.LengthExponent unconditionally for
-        # every IfcSIUnit, which crashed project loading for any linked file.
-        # See the PR discussion for a standalone reproduction script.
+        # Bonsai's "linked project" large-model workflow) and raises there
+        # instead of returning an IfcDimensionalExponents entity.
+        # Test ensures `calculate_unit_scale` doesn't rely on derived attribute computation.
         ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcProject")
         length = ifcopenshell.api.unit.add_si_unit(self.file, unit_type="LENGTHUNIT")
         ifcopenshell.api.unit.assign_unit(self.file, units=[length])
@@ -345,7 +343,6 @@ class TestCalculateUnitScaleOnLinkedFile(test.bootstrap.IFC4):
 
         try:
             linked_file = ifcopenshell.open(str(tmp_file))
-            assert linked_file.by_type("IfcSIUnit")[0].Dimensions is None
             assert subject.calculate_unit_scale(linked_file, "LENGTHUNIT") == 1.0
         finally:
             if isinstance(linked_file, ifcopenshell.sqlite):