test: Implement post-test database cleanup in workflow history test

tests/test_workflow_history.py

@@ -8,12 +8,17 @@ from sqlalchemy import delete
 def history_manager():
     settings = Settings()
     manager = HistoryManager(settings.history_db_url)
-    with manager.get_session() as session:
-        session.execute(delete(Plot))
-        session.execute(delete(Message))
-        session.execute(delete(Conversation))
-        session.execute(delete(User))
-    return manager
+    
+    def cleanup():
+        with manager.get_session() as session:
+            session.execute(delete(Plot))
+            session.execute(delete(Message))
+            session.execute(delete(Conversation))
+            session.execute(delete(User))
+    
+    cleanup()
+    yield manager
+    cleanup()
 
 def test_full_history_workflow(history_manager):
     # 1. Create and Authenticate User