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/unit/core/materials/test_concrete04.py

@@ -136,6 +136,7 @@ class TestConcrete04JsonRoundTrip:
     def test_old_osmodel_without_concrete04_loads_cleanly(self) -> None:
         """An osmodel payload that doesn't mention Concrete04 is unaffected."""
         from pathlib import Path
+
         from otko.services import load_project
 
         osmodel = Path(__file__).parents[4] / "examples" / "cantilever.osmodel"

tests/unit/test_diagram_renderer.py

@@ -15,21 +15,21 @@ import pytest
 # Force pyvista off-screen before any pyvista import in this module's chain.
 os.environ.setdefault("PYVISTA_OFF_SCREEN", "true")
 
-import pyvista as pv  # noqa: E402
+import pyvista as pv
 
-from otko.core import (  # noqa: E402
+from otko.core import (
     ElasticBeamColumn,
     ElasticSection,
     Node,
     Project,
 )
-from otko.services.element_forces import (  # noqa: E402
+from otko.services.element_forces import (
     DiagramData,
     ForceComponent,
     extract_diagram_data,
 )
-from otko.services.results import StaticResults  # noqa: E402
-from otko.views.canvas3d.diagram_renderer import DiagramRenderer  # noqa: E402
+from otko.services.results import StaticResults
+from otko.views.canvas3d.diagram_renderer import DiagramRenderer
 
 pv.OFF_SCREEN = True
 

tests/unit/test_elements.py

@@ -6,8 +6,8 @@ import pytest
 from pydantic import TypeAdapter, ValidationError
 
 from otko.core import (
-    Element,
     ElasticBeamColumn,
+    Element,
     ForceBeamColumn,
     TrussElement,
     ZeroLengthElement,

tests/unit/test_grid_system.py

@@ -2,7 +2,6 @@
 
 from __future__ import annotations
 
-import math
 from pathlib import Path
 
 import pytest
@@ -174,7 +173,7 @@ def test_coord_system_round_trip_world_local() -> None:
     p_local = (1.5, -0.5, 2.0)
     p_world = cs.local_to_world(p_local)
     p_back = cs.world_to_local(p_world)
-    for a, b in zip(p_local, p_back):
+    for a, b in zip(p_local, p_back, strict=False):
         assert a == pytest.approx(b, abs=1e-9)
 
 

tests/unit/test_phase8a.py

@@ -8,7 +8,6 @@
 
 from __future__ import annotations
 
-import numpy as np
 import pytest
 
 from otko.commands.nodes import SetMassCommand

tests/unit/test_phase9.py

@@ -10,9 +10,7 @@ import pytest
 from otko.core import (
     AggregatorDOF,
     CircularPatch,
-    ElasticSection,
     FiberSection,
-    Fibre,
     Project,
     RectangularPatch,
     SectionAggregator,
@@ -155,7 +153,7 @@ def test_Iz_of_rect_matches_analytical() -> None:
 
 # ── Round-trip persistence ───────────────────────────────────────────
 def test_fiber_section_with_patches_round_trips(tmp_path) -> None:  # type: ignore[no-untyped-def]
-    from otko.core import Node, ElasticBeamColumn
+    from otko.core import ElasticBeamColumn, Node
 
     p = Project(
         nodes=[Node(id=1, coords=(0, 0, 0)), Node(id=2, coords=(1, 0, 0))],

tests/unit/test_renderer.py

@@ -5,9 +5,9 @@ from __future__ import annotations
 import pytest
 
 pv = pytest.importorskip("pyvista")
-import numpy as np  # noqa: E402
+import numpy as np
 
-from otko.core import (  # noqa: E402
+from otko.core import (
     ElasticBeamColumn,
     ElasticSection,
     LinearTimeSeries,
@@ -18,8 +18,8 @@ from otko.core import (  # noqa: E402
     Steel01,
     TrussElement,
 )
-from otko.services.deformation import DeformationSource  # noqa: E402
-from otko.views.canvas3d.model_renderer import (  # noqa: E402
+from otko.services.deformation import DeformationSource
+from otko.views.canvas3d.model_renderer import (
     ModelRenderer,
     RendererMode,
     _classify_support,

tests/unit/test_unit_labels.py

@@ -2,8 +2,6 @@
 
 from __future__ import annotations
 
-import pytest
-
 from otko.core import UnitSystem, labels_for
 
 

tests/unit/test_zerolength_section.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import pytest
+from pydantic import ValidationError
 
 from otko.core import (
     ElasticSection,
@@ -21,7 +22,7 @@ def test_zero_length_section_schema_defaults() -> None:
 
 
 def test_zero_length_section_rejects_extra_fields() -> None:
-    with pytest.raises(Exception):
+    with pytest.raises(ValidationError):
         ZeroLengthSectionElement(
             id=1,
             nodes=(1, 2),