test: verify delayed prompt injection

Fix pre-existing ty error in action_new_session by typing the sandbox
element as SandboxPolicy | None instead of object.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_tui.py

@@ -1782,7 +1782,9 @@ async def test_schedule_prompt_action_calls_service_and_refreshes():
         with patch.object(app, "notify", side_effect=capture_notify):
             app.action_schedule_prompt()
             await pilot.pause()
-            app.screen.dismiss(("continue", __import__("datetime").timedelta(minutes=30)))
+            app.screen.dismiss(
+                ("continue", __import__("datetime").timedelta(minutes=30))
+            )
             await pilot.pause()
             await app.workers.wait_for_complete()