Implement OpenAI Responses API [1/N] (#20504)

Signed-off-by: Woosuk Kwon <woosuk.kwon@berkeley.edu>

tests/v1/entrypoints/openai/responses/conftest.py

@@ -0,0 +1,32 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+import pytest
+import pytest_asyncio
+
+from tests.utils import RemoteOpenAIServer
+
+# Use a small reasoning model to test the responses API.
+MODEL_NAME = "Qwen/Qwen3-0.6B"
+
+
+@pytest.fixture(scope="module")
+def default_server_args():
+    return [
+        "--max-model-len",
+        "8192",
+        "--enforce-eager",  # For faster startup.
+        "--reasoning-parser",
+        "deepseek_r1",
+    ]
+
+
+@pytest.fixture(scope="module")
+def server(default_server_args):
+    with RemoteOpenAIServer(MODEL_NAME, default_server_args) as remote_server:
+        yield remote_server
+
+
+@pytest_asyncio.fixture
+async def client(server):
+    async with server.get_async_client() as async_client:
+        yield async_client