[Tests] Verify gpt_oss package is installed in harmony tests (#29336)

Signed-off-by: Nick Hill <nhill@redhat.com>

tests/entrypoints/openai/test_response_api_with_harmony.py

 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
-
+import importlib
 import json
 import time
 
@@ -35,6 +35,10 @@ GET_WEATHER_SCHEMA = {
 
 @pytest.fixture(scope="module")
 def server():
+    assert importlib.util.find_spec("gpt_oss") is not None, (
+        "Harmony tests require gpt_oss package to be installed"
+    )
+
     args = ["--enforce-eager", "--tool-server", "demo", "--max_model_len", "5000"]
     env_dict = dict(
         VLLM_ENABLE_RESPONSES_API_STORE="1",