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_example.py

@@ -6,8 +6,8 @@ import pytest
 
 pytest.importorskip("openseespy")
 
-from otko.services import load_project  # noqa: E402
-from otko.services.opensees_runner import OpenSeesRunner  # noqa: E402
+from otko.services import load_project
+from otko.services.opensees_runner import OpenSeesRunner
 
 pytestmark = pytest.mark.slow
 
@@ -15,13 +15,13 @@ pytestmark = pytest.mark.slow
 def test_moment_curvature_example_round_trips_and_converges(tmp_path) -> None:  # type: ignore[no-untyped-def]
     """build_moment_curvature() → save → load → run → expected shape."""
     from examples.moment_curvature import (
-        build_moment_curvature,
         COL_DEPTH,
         COVER,
         E_STEEL,
         FY,
         MU,
         NUM_INCR,
+        build_moment_curvature,
     )
 
     proj = build_moment_curvature()