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_material_form_ranges.py

@@ -10,7 +10,7 @@ import pytest
 
 pytest.importorskip("PySide6")
 
-from otko.views.dialogs.material_forms import (  # noqa: E402
+from otko.views.dialogs.material_forms import (
     Concrete01Form,
     Concrete02Form,
     Steel01Form,