[Misc] Fix Current vLLM config is not set. warnings, assert to avoid issues in the future (#31747)

Signed-off-by: Lucas Wilkinson <lwilkins@redhat.com>
Signed-off-by: Lucas Wilkinson <LucasWilkinson@users.noreply.github.com>
Co-authored-by: Luka Govedič <ProExpertProg@users.noreply.github.com>

tests/conftest.py

@@ -189,6 +189,17 @@ def dist_init():
     cleanup_dist_env_and_memory()
 
 
+@pytest.fixture
+def default_vllm_config():
+    """Set a default VllmConfig for tests that directly test CustomOps or pathways
+    that use get_current_vllm_config() outside of a full engine context.
+    """
+    from vllm.config import VllmConfig, set_current_vllm_config
+
+    with set_current_vllm_config(VllmConfig()):
+        yield
+
+
 @pytest.fixture()
 def should_do_global_cleanup_after_test(request) -> bool:
     """Allow subdirectories to skip global cleanup by overriding this fixture.