refactor: centralize llm provider transport

app/tests/test_review_assistant.py

@@ -31,7 +31,7 @@ class ReviewAssistantStreamingTests(unittest.TestCase):
                 b'data: [DONE]\n',
             ]
         )
-        with patch("assistant_agent.urllib.request.urlopen", return_value=response):
+        with patch("backend.llm.transport.urllib.request.urlopen", return_value=response):
             chunks = list(
                 stream_review_assistant({}, "question", [], "key", "https://example.test/v1", "model")
             )
@@ -39,7 +39,7 @@ class ReviewAssistantStreamingTests(unittest.TestCase):
 
     def test_empty_stream_is_rejected(self):
         response = StreamingResponse([b"data: [DONE]\n"])
-        with patch("assistant_agent.urllib.request.urlopen", return_value=response):
+        with patch("backend.llm.transport.urllib.request.urlopen", return_value=response):
             with self.assertRaises(ReviewAssistantError):
                 list(
                     stream_review_assistant({}, "question", [], "key", "https://example.test/v1", "model")
@@ -54,7 +54,7 @@ class ReviewAssistantStreamingTests(unittest.TestCase):
                 b"data: [DONE]\n",
             ]
         )
-        with patch("assistant_agent.urllib.request.urlopen", return_value=response):
+        with patch("backend.llm.transport.urllib.request.urlopen", return_value=response):
             chunks = list(
                 stream_review_assistant(
                     {}, "question", [], "key", "https://example.test/v1", "model"