Bind run_state.get_prior result before chaining .content access

Mypy on CI flagged ``second_run_state.get_prior(...).content`` because
``get_prior`` returns Optional[AgentResponse]. The preceding ``is not
None`` assert doesn't narrow the next call's return type. Bind the
result first, assert non-None, then access .content.

Author: dannon

test/unit/app/test_agents.py

@@ -719,8 +719,9 @@ def get_agent_side_effect(agent_type, deps):
         assert first_run_state is second_run_state
 
         # First agent saw an empty run_state; second agent saw history recorded
-        assert second_run_state.get_prior("history") is not None
-        assert second_run_state.get_prior("history").content == "History summary content"
+        history_prior = second_run_state.get_prior("history")
+        assert history_prior is not None
+        assert history_prior.content == "History summary content"
 
     @pytest.mark.asyncio
     async def test_orchestrator_sequential_passes_original_query(self):