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/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))],