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_moment_curvature.py

@@ -8,13 +8,11 @@ behaviour is exercised by the Phase 9 pushover tests.
 
 from __future__ import annotations
 
-import math
-
 import pytest
 
 pytest.importorskip("openseespy")
 
-from otko.core import (  # noqa: E402
+from otko.core import (
     ElasticUniaxial,
     FiberSection,
     LinearTimeSeries,
@@ -26,7 +24,7 @@ from otko.core import (  # noqa: E402
     StaticCase,
     ZeroLengthSectionElement,
 )
-from otko.services.opensees_runner import OpenSeesRunner  # noqa: E402
+from otko.services.opensees_runner import OpenSeesRunner
 
 pytestmark = pytest.mark.slow
 
@@ -134,7 +132,7 @@ def test_pushover_drives_rotation_for_moment_curvature() -> None:
 
     # Every (κ, M) point must satisfy M = E·I·κ (1 % tolerance allows
     # for the ~20-fibre discretisation of the rectangular section).
-    for kappa, moment in zip(result.control_disp, result.base_shear):
+    for kappa, moment in zip(result.control_disp, result.base_shear, strict=False):
         if abs(kappa) < 1e-12:
             continue
         expected_M = E * I * kappa