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_gravity.py

@@ -6,8 +6,8 @@ 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
 
@@ -22,7 +22,7 @@ def test_rc_frame_gravity_matches_opensees_reference(tmp_path) -> None:  # type:
     The column axial force is 180 kip compression (from the 180 kip
     load stepped onto each top node).
     """
-    from examples.rc_frame_gravity import build_rc_frame_gravity, P_LOAD
+    from examples.rc_frame_gravity import P_LOAD, build_rc_frame_gravity
 
     proj = build_rc_frame_gravity()
     proj.validate_references()