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_rc_frame_earthquake.py

@@ -9,8 +9,8 @@ import pytest
 
 pytest.importorskip("openseespy")
 
-from otko.services import load_project, save_project  # noqa: E402
-from otko.services.opensees_runner import OpenSeesRunner  # noqa: E402
+from otko.services import load_project, save_project
+from otko.services.opensees_runner import OpenSeesRunner
 
 pytestmark = pytest.mark.slow
 
@@ -18,9 +18,8 @@ pytestmark = pytest.mark.slow
 def test_rc_frame_earthquake_runs_and_has_oscillatory_response(tmp_path) -> None:  # type: ignore[no-untyped-def]
     """Synthetic ground motion produces bounded, oscillatory response."""
     from examples.rc_frame_earthquake import (
-        build_rc_frame_earthquake,
-        DT,
         N_PTS,
+        build_rc_frame_earthquake,
     )
 
     proj = build_rc_frame_earthquake()