refactor: remove legacy assistant compatibility

backend/tests/test_pipeline_knowledge.py

@@ -46,10 +46,6 @@ class KnowledgeToolDescriptionTest(unittest.TestCase):
         processor = KnowledgeRetrievalProcessor(None)
         messages = [
             {"role": "assistant", "content": "你好"},
-            {
-                "role": "developer",
-                "content": f"{KNOWLEDGE_CONTEXT_MARKER}\n旧检索结果",
-            },
         ]
 
         processor._set_context(
@@ -59,7 +55,6 @@ class KnowledgeToolDescriptionTest(unittest.TestCase):
 
         self.assertEqual(messages[0]["role"], "system")
         self.assertIn("新检索结果", messages[0]["content"])
-        self.assertFalse(any(message["role"] == "developer" for message in messages))
 
 
 class UserTurnRoutingProcessorTest(unittest.IsolatedAsyncioTestCase):