fix(simulation): cache idle worker state

src/k1link/simulation/worker_agent.py

@@ -637,9 +637,8 @@ class SimulationWorkerAgent:
         )
 
     def _active_run(self) -> QualificationRun | None:
-        cached: QualificationRun | None = getattr(self, "_active_run_cache", None)
-        if cached is not None:
-            return cached
+        if hasattr(self, "_active_run_cache"):
+            return self._active_run_cache
         active = tuple(run for run in self.store.list_runs() if run.state in ACTIVE_STATES)
         if len(active) > 1:
             raise WorkerAgentError("multiple active qualification runs violate worker authority")