fix: prevent None reply in collection flow and harden response fallback

tests/test_regression_pipeline.py

@@ -337,6 +337,33 @@ class RegressionPipelineTest(unittest.IsolatedAsyncioTestCase):
         self.assertEqual(st.pending_image_urls, ["u1", "u2"])
         self.assertEqual(st.pending_requirements, ["r1"])
 
+    async def test_collection_reply_never_returns_none(self):
+        os.environ["AI_DYNAMIC_COLLECTION_REPLIES"] = "true"
+        agent = CustomerServiceAgent()
+        agent.agent_natural_reply.run = AsyncMock(side_effect=RuntimeError("mock ai fail"))
+        st = agent._get_conversation_state(self.customer_id)
+        msg = CustomerMessage(
+            msg_id="m-collection-fallback",
+            acc_id="test_shop",
+            msg="收到没",
+            from_id=self.customer_id,
+            from_name="t",
+            cy_id=self.customer_id,
+            acc_type="AliWorkbench",
+            msg_type=0,
+            cy_name="t",
+            goods_name="专业找图",
+            goods_order="",
+        )
+        reply = await agent._render_collection_reply_with_ai(
+            message=msg,
+            state=st,
+            scene="collect_ack",
+            intent_hint="确认已收到图片",
+            fallback="图片收到了，你继续发就行。",
+        )
+        self.assertEqual(reply, "图片收到了，你继续发就行。")
+
     def tearDown(self):
         db.clear_pending_quote_state(self.customer_id)