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_elastic_frame.py

@@ -8,9 +8,9 @@ import pytest
 
 pytest.importorskip("openseespy")
 
-from otko.core import ModalCase, StaticCase  # noqa: E402
-from otko.services import load_project, save_project  # noqa: E402
-from otko.services.opensees_runner import OpenSeesRunner  # noqa: E402
+from otko.core import ModalCase, StaticCase
+from otko.services import load_project, save_project
+from otko.services.opensees_runner import OpenSeesRunner
 
 pytestmark = pytest.mark.slow
 
@@ -29,7 +29,6 @@ def _reload(proj, tmp_path):  # type: ignore[no-untyped-def]
 def test_elastic_frame_gravity_reactions(tmp_path) -> None:  # type: ignore[no-untyped-def]
     """ΣFy at base = total applied gravity (distributed w × beam × floors)."""
     from examples.elastic_frame import (
-        BAY,
         LOAD_F1,
         LOAD_F2,
         LOAD_F3,
@@ -70,7 +69,6 @@ def test_elastic_frame_gravity_reactions(tmp_path) -> None:  # type: ignore[no-u
 def test_elastic_frame_gravity_plus_lateral_reactions(tmp_path) -> None:  # type: ignore[no-untyped-def]
     """ΣFx at base must equal -(lateral applied) within PDelta tolerance."""
     from examples.elastic_frame import (
-        BAY,
         LOAD_F1,
         LOAD_F2,
         LOAD_F3,
@@ -120,5 +118,5 @@ def test_elastic_frame_modal_periods(tmp_path) -> None:  # type: ignore[no-untyp
     # 0.1930, 0.1562, 0.130 s. Our solve nails these within 1.5%.
     expected = [1.040, 0.3526, 0.1930, 0.1562, 0.130]
     periods = [2.0 * math.pi / math.sqrt(v) for v in r.eigenvalues]
-    for i, (T, T_ref) in enumerate(zip(periods, expected), start=1):
-        assert T == pytest.approx(T_ref, rel=0.02), f"T{i} = {T:.4f} s, reference {T_ref:.4f} s"
+    for i, (T, T_ref) in enumerate(zip(periods, expected, strict=False), start=1):
+        assert pytest.approx(T_ref, rel=0.02) == T, f"T{i} = {T:.4f} s, reference {T_ref:.4f} s"