bim.reassign_class - fix issue in ifc2x3 when selected product class would be ignored

E.g. you select IfcSlab and change it to IfcRoof - since there is not IfcRoofType in IFC2X3 it would figure the matching product type is IfcBeamType (which is confusing too but that's another subject) and change this slab's type object to IfcBeamType which consequently change IfcSlab to IfcBeam instead of IfcRoof that was selected originally.

Noticed investigating #5918

src/ifcopenshell-python/test/api/root/test_reassign_class.py

@@ -198,4 +198,25 @@ class TestReassignClass(test.bootstrap.IFC4):
 
 
 class TestReassignClassIFC2X3(test.bootstrap.IFC2X3, TestReassignClass):
-    pass
+    def test_providing_occurrence_class(self):
+        element_type = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcWallType")
+        element1 = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcWall")
+        ifcopenshell.api.type.assign_type(self.file, related_objects=[element1], relating_type=element_type)
+        element2 = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcWall")
+        ifcopenshell.api.type.assign_type(self.file, related_objects=[element2], relating_type=element_type)
+
+        new_element_type = ifcopenshell.api.root.reassign_class(
+            self.file,
+            product=element_type,
+            ifc_class="IfcBuildingElementProxyType",
+            occurrence_class="IfcRoof",
+        )
+        assert new_element_type.is_a("IfcBuildingElementProxyType")
+        occurrences = ifcopenshell.util.element.get_types(new_element_type)
+        assert len(occurrences) == 2
+        # Assign IfcRoof instead of IfcBuildingElementProxy.
+        assert all(o.is_a("IfcRoof") for o in occurrences)
+
+        # original clases are gone
+        assert len(self.file.by_type("IfcWall")) == 0
+        assert len(self.file.by_type("IfcWallType")) == 0