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/services/test_element_forces.py

@@ -19,7 +19,6 @@ from otko.services.element_forces import (
 )
 from otko.services.results import StaticResults
 
-
 # ──────────────────────────────────────────────────────────────────────
 # Fixtures: a tiny 3D project with 2 elements + canned force results.
 # ──────────────────────────────────────────────────────────────────────

tests/services/test_runner_translation.py

@@ -23,7 +23,6 @@ from otko.core import (
     Node,
     PlainLoadPattern,
     Project,
-    StaticCase,
     Steel01,
     TrussElement,
     ZeroLengthElement,

tests/services/test_section_bbox.py

@@ -10,7 +10,6 @@ from otko.core import (
     CircularPatch,
     ElasticSection,
     FiberSection,
-    Project,
     RectangularPatch,
     StraightLayer,
 )