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_run_analysis.py

@@ -8,11 +8,11 @@ import pytest
 
 pytest.importorskip("PySide6")
 
-from PySide6.QtCore import QObject, Signal  # noqa: E402
+from PySide6.QtCore import QObject, Signal
 
-from otko.core import LinearTimeSeries, PlainLoadPattern, Project, TransientCase  # noqa: E402
-from otko.viewmodels import ProjectViewModel  # noqa: E402
-from otko.views.dialogs.run_analysis import RunAnalysisDialog  # noqa: E402
+from otko.core import LinearTimeSeries, PlainLoadPattern, TransientCase
+from otko.viewmodels import ProjectViewModel
+from otko.views.dialogs.run_analysis import RunAnalysisDialog
 
 
 class _FakeRunner(QObject):