refactor: centralize llm provider transport

app/tests/test_mentor_stream.py

@@ -44,7 +44,7 @@ class MentorStreamTests(unittest.TestCase):
             captured["accept"] = request.headers.get("Accept")
             return FakeStreamResponse(self.lines)
 
-        with patch("mentor_agent.urllib.request.urlopen", side_effect=open_request):
+        with patch("backend.llm.transport.urllib.request.urlopen", side_effect=open_request):
             chunks = list(
                 stream_with_mentor(
                     self.skill, {"data_trade_date": "20260723"}, "怎么看？", [],
@@ -58,7 +58,7 @@ class MentorStreamTests(unittest.TestCase):
 
     def test_non_streaming_compatibility_wrapper_collects_chunks(self):
         with patch(
-            "mentor_agent.urllib.request.urlopen",
+            "backend.llm.transport.urllib.request.urlopen",
             return_value=FakeStreamResponse(self.lines),
         ):
             result = chat_with_mentor(
@@ -74,7 +74,7 @@ class MentorStreamTests(unittest.TestCase):
             b"data: [DONE]\n",
         ]
         with patch(
-            "mentor_agent.urllib.request.urlopen",
+            "backend.llm.transport.urllib.request.urlopen",
             return_value=FakeStreamResponse(lines),
         ):
             chunks = list(
@@ -92,7 +92,7 @@ class MentorStreamTests(unittest.TestCase):
             b"data: [DONE]\n",
         ]
         with patch(
-            "mentor_agent.urllib.request.urlopen",
+            "backend.llm.transport.urllib.request.urlopen",
             return_value=FakeStreamResponse(lines),
         ):
             chunks = list(