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/integration/test_runner_modal.py

@@ -15,16 +15,16 @@ import math
 
 import pytest
 
-ops = pytest.importorskip("openseespy.opensees")  # noqa: F401
+ops = pytest.importorskip("openseespy.opensees")
 
-from otko.core import (  # noqa: E402
+from otko.core import (
     ElasticBeamColumn,
     ElasticSection,
     ModalCase,
     Node,
     Project,
 )
-from otko.services import OpenSeesRunner  # noqa: E402
+from otko.services import OpenSeesRunner
 
 pytestmark = pytest.mark.slow