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/gui/test_draw_node_tool.py

@@ -10,10 +10,10 @@ import pytest
 
 pytest.importorskip("PySide6")
 
-from otko.core import GridSystem, Node, Project  # noqa: E402
-from otko.viewmodels import ProjectViewModel  # noqa: E402
-from otko.views.canvas3d.selection import SelectionState  # noqa: E402
-from otko.views.tools.draw_node import (  # noqa: E402
+from otko.core import GridSystem, Node
+from otko.viewmodels import ProjectViewModel
+from otko.views.canvas3d.selection import SelectionState
+from otko.views.tools.draw_node import (
     DrawNodeTool,
     _snap_to_grid,
 )