test(engine): expect EngineConnectionError for vllm 404 (fix main CI) (#530)

#463 made the OpenAI-compatible engine wrap upstream HTTP errors (incl.
404) in EngineConnectionError with an actionable message, but
test_invalid_model_404 still asserted the raw httpx.HTTPStatusError, so it
broke on main once #463 landed (#463 was a stale fork PR with no CI, so it
wasn't caught pre-merge). Expect EngineConnectionError now, asserting the
httpx.HTTPStatusError is preserved as the chained cause. Whole tests/engine
suite is green again.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/engine/test_vllm_models.py

@@ -204,10 +204,14 @@ class TestVLLMErrors:
         respx_mock.post(f"{VLLM_HOST}/v1/chat/completions").mock(
             return_value=httpx.Response(404, json={"error": "model not found"})
         )
-        with pytest.raises(httpx.HTTPStatusError):
+        # The OpenAI-compatible engine wraps upstream HTTP errors (incl. 404)
+        # in EngineConnectionError with an actionable message (see #463); the
+        # raw httpx.HTTPStatusError is the chained cause.
+        with pytest.raises(EngineConnectionError) as exc_info:
             engine.generate(
                 [Message(role=Role.USER, content="Hi")], model="nonexistent"
             )
+        assert isinstance(exc_info.value.__cause__, httpx.HTTPStatusError)
 
     def test_timeout_raises_connection_error(self) -> None:
         engine = _make_engine()