vllm plugin: skip backend tests without vllm

Backend selection imports salm.backends, which depends on vLLM symbols. Guard those tests on vLLM availability so CPU SpeechLM2 shards without vLLM skip them like the other plugin runtime tests.

Made-with: Cursor
Signed-off-by: Dongji Gao <dongjig@nvidia.com>

Author: DongjiGao

tests/collections/speechlm2/test_vllm_plugin.py

@@ -210,7 +210,7 @@ def test_unknown_attr_raises(self):
             _ = cfg.nonexistent_attribute_xyz
 
 
-@pytest.mark.skipif(not _HAS_CONFIG, reason="NeMoSpeechLMConfig not available")
+@pytest.mark.skipif(not (_HAS_CONFIG and _HAS_VLLM), reason="NeMoSpeechLMConfig or vLLM not available")
 class TestBackendSelection:
     """Tests for ``backends.make_backend`` dispatch on hybrid/transformer configs."""