fix: fix test race condition

ref: N25B-301

test/unit/core/test_agent_system.py

@@ -33,14 +33,16 @@ async def test_agent_lifecycle():
 
     # Test background task
     async def dummy_task():
-        await asyncio.sleep(0.01)
+        pass
 
-    await agent.add_behavior(dummy_task())
-    assert len(agent._tasks) > 0
+    task = agent.add_behavior(dummy_task())
+    assert task in agent._tasks
+
+    await task
 
     # Wait for task to finish
-    await asyncio.sleep(0.02)
-    assert len(agent._tasks) == 2  # message handling tasks are running
+    assert task not in agent._tasks
+    assert len(agent._tasks) == 2  # message handling tasks are still running
 
     await agent.stop()
     assert agent._running is False