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_transient.py

@@ -12,10 +12,10 @@ import math
 import numpy as np
 import pytest
 
-ops = pytest.importorskip("openseespy.opensees")  # noqa: F401
-h5py = pytest.importorskip("h5py")  # noqa: F401
+ops = pytest.importorskip("openseespy.opensees")
+h5py = pytest.importorskip("h5py")
 
-from otko.core import (  # noqa: E402
+from otko.core import (
     ConstantTimeSeries,
     ElasticBeamColumn,
     ElasticSection,
@@ -27,7 +27,7 @@ from otko.core import (  # noqa: E402
     StaticCase,
     TransientCase,
 )
-from otko.services import OpenSeesRunner  # noqa: E402
+from otko.services import OpenSeesRunner
 
 pytestmark = pytest.mark.slow
 
@@ -51,7 +51,6 @@ def test_sdof_free_vibration_matches_cosine(tmp_path) -> None:  # type: ignore[n
     # (already done), then verify dt-step Newmark integration of free
     # vibration starting from a static IC.
     F0 = 100.0
-    u0 = F0 / k
 
     project = Project(
         ndm=2,