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_basic_truss.py

@@ -11,7 +11,7 @@ import pytest
 
 pytest.importorskip("openseespy")
 
-from otko.services.opensees_runner import OpenSeesRunner  # noqa: E402
+from otko.services.opensees_runner import OpenSeesRunner
 
 pytestmark = pytest.mark.slow
 
@@ -25,7 +25,7 @@ def test_basic_truss_matches_opensees_tcl_reference() -> None:
     a linear rescale of the Tcl model — any deviation would flag a
     real solver/translation issue.
     """
-    from examples.basic_truss import build_basic_truss, IN_TO_M
+    from examples.basic_truss import IN_TO_M, build_basic_truss
 
     proj = build_basic_truss()
     runner = OpenSeesRunner(proj)
@@ -45,6 +45,7 @@ def test_basic_truss_matches_opensees_tcl_reference() -> None:
 def test_basic_truss_round_trips(tmp_path) -> None:  # type: ignore[no-untyped-def]
     """The example project must survive save/load without any information loss."""
     from examples.basic_truss import build_basic_truss
+
     from otko.services import load_project, save_project
 
     p = build_basic_truss()