test: install pyyaml and isolate vllm config file in tests

after rebasing onto main, get_engine_args() loads a vllm-style config via
PyYAML (a transitive vllm dep). vllm is stubbed in tests, so add pyyaml
explicitly and point VLLM_CONFIG_FILE at a nonexistent path so no stray
config is picked up.

tests/requirements.txt

@@ -1,3 +1,6 @@
 # Test-only dependencies. vllm/torch are stubbed in conftest.py when absent,
 # so the unit tests run on a plain CPU runner without the GPU image.
 pytest>=8,<10
+# get_engine_args() reads a vLLM-style config via PyYAML (a transitive vllm dep
+# at runtime); install it explicitly here since vllm itself is stubbed.
+pyyaml

tests/test_engine_args.py

@@ -94,6 +94,8 @@ class TestGetEngineArgsServedName:
         # Avoid the network branch in _resolve_max_model_len.
         monkeypatch.setenv("MAX_NUM_BATCHED_TOKENS", "2048")
         monkeypatch.delenv("SERVED_MODEL_NAME", raising=False)
+        # Don't pick up a stray vLLM config file from the environment.
+        monkeypatch.setenv("VLLM_CONFIG_FILE", "/nonexistent-vllm-config.yaml")
 
     @requires_case_sensitive_fs
     def test_served_name_is_repo_id_when_path_rewritten(self, hf_cache, monkeypatch):