style: resolve the remaining ruff findings

Auto-fixes (UP037, UP038, F401, I001, SIM105, RUF100) plus hand-fixes:
drop the now-unused `Union` imports, annotate mutable class attributes
with ClassVar, raise ValidationError instead of blind Exception in the
zero-length test, delete two dead locals, and bind the per-iteration
grid-builder closures to their loop variables (B023).

tests/gui/test_property_forms.py

@@ -10,13 +10,10 @@ import pytest
 
 pytest.importorskip("PySide6")
 
-from otko.core import (  # noqa: E402
-    Concrete01,
+from otko.core import (
     Concrete02,
     ElasticIsotropic,
-    ElasticPP,
     ElasticSection,
-    ElasticUniaxial,
     Steel01,
     Steel02,
 )
@@ -100,10 +97,14 @@ def test_elastic_section_form_round_trip(qtbot) -> None:  # type: ignore[no-unty
 def test_form_for_dispatches_by_type(qtbot) -> None:  # type: ignore[no-untyped-def]
     from otko.views.dialogs.material_forms import (
         Steel01Form,
+    )
+    from otko.views.dialogs.material_forms import (
         form_for as material_form_for,
     )
     from otko.views.dialogs.section_forms import (
         ElasticSectionForm,
+    )
+    from otko.views.dialogs.section_forms import (
         form_for as section_form_for,
     )