Fix assigning a plain material to an occurrence as a layer set

Assigning a material to an occurrence with a set material type has raised
"IfcMaterial cannot be assiged as a IfcMaterialLayerSetUsage" since the
default changed to assigning usages to occurrences. The type is upgraded to a
usage but the material is passed on unchanged, and material.assign_material
only accepts a material for a usage when that material is already the set,
whereas the Object Materials dropdown gives us a plain IfcMaterial. Pass
nothing in that case and let the API make the set, as it does when asked for
a usage with no material.

Look the set up past the usage afterwards, so the material the user picked is
added to it. get_material returns the usage, which is not a material set, so
neither branch of the repair below matched and the picked material was
dropped, leaving the set empty.

This is a stopgap and is commented as such: the real problem is that
assign_material builds sets with no items in them and ignores the material it
was given, which is not valid IFC and leaves callers patching up after it.

Also register "I evaluate expression" as a Then step. It has only ever been a
Given and a When, so the last line of the scenario covering this could never
run; it is the only Then of its kind in the suite.

test/bim goes from 16 failures to 15, with none introduced.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/bonsai/test/bim/test_feature.py

@@ -1000,6 +1000,7 @@ def i_click_button_and_expect_error_error_msg(button, error_msg):
 
 @given(parsers.parse('I evaluate expression "{expression}"'))
 @when(parsers.parse('I evaluate expression "{expression}"'))
+@then(parsers.parse('I evaluate expression "{expression}"'))
 def i_evaluate_expression(expression):
     expression = replace_variables(expression)
     exec(expression)