test(brains): cover Cohere upstream-error raise

Regression test asserting a 429 error body raises RuntimeError with the
status and message, matching test_stream_raises_on_upstream_error for the
anthropic adapter. Also drop a pre-existing unused ChatChunk import in the
touched file.

Author: Zawwarsami16

tests/test_brains_together_mistral_cohere.py

@@ -6,7 +6,6 @@
 import httpx
 import pytest
 
-from zhub.brains.base import ChatChunk
 from zhub.brains.together import TogetherAdapter
 from zhub.brains.mistral import MistralAdapter
 from zhub.brains.cohere import CohereAdapter
@@ -145,3 +144,48 @@ async def test_cohere_stream_parses_v2_event_shape():
     assert body["messages"][-1] == {"role": "user", "content": "x"}
     headers = fake.last_call["headers"]
     assert headers["Authorization"] == "Bearer co_test"
+
+
+class _FakeErrorStream:
+    def __init__(self, status_code, body):
+        self.status_code = status_code
+        self._body = body.encode()
+
+    async def aiter_lines(self):
+        yield self._body.decode()
+
+    async def aread(self):
+        return self._body
+
+    async def __aenter__(self):
+        return self
+
+    async def __aexit__(self, *exc):
+        return None
+
+
+class _FakeErrorClient:
+    def __init__(self, status_code, body):
+        self._status = status_code
+        self._body = body
+
+    def stream(self, method, url, **kw):
+        return _FakeErrorStream(self._status, self._body)
+
+    async def aclose(self):
+        pass
+
+
+@pytest.mark.asyncio
+async def test_cohere_stream_raises_on_upstream_error():
+    """A non-2xx body is a JSON error, not Cohere's newline-delimited events —
+    the adapter must raise rather than end the stream silently with no content
+    (matching the anthropic / openai-compat adapters)."""
+    fake = _FakeErrorClient(429, '{"message":"rate limit exceeded"}')
+    a = CohereAdapter(api_key="co_test", model="command-r-plus-08-2024", http=fake)
+    with pytest.raises(RuntimeError) as ei:
+        async for _ in a.stream([{"role": "user", "content": "hi"}]):
+            pass
+    msg = str(ei.value)
+    assert "429" in msg
+    assert "rate limit exceeded" in msg