feat(chat): stop an in-flight answer — Send becomes Stop, the partial is kept and persisted, the model stream is torn down

tests/unit/test_llm_client.py

@@ -289,6 +289,7 @@ def _chunk(
 class _FakeChatStream:
     def __init__(self, chunks: list) -> None:
         self._chunks = list(chunks)
+        self.close_calls = 0
 
     def __aiter__(self):
         self._i = 0
@@ -301,6 +302,11 @@ class _FakeChatStream:
         self._i += 1
         return chunk
 
+    async def close(self) -> None:
+        """The openai SDK ``AsyncStream.close()`` (phase 48): ``chat_stream``
+        awaits it on every exit after a successful ``create()``."""
+        self.close_calls += 1
+
 
 class _FakeCompletion:
     """One fake non-streaming ChatCompletion (``choices[].message`` shape).