Add reference counting to ModelInstance for parallel test safety (#16672)

sgl-model-gateway/e2e_test/fixtures/pool.py

@@ -151,6 +151,7 @@ def model_client(request: pytest.FixtureRequest, model_pool: "ModelPool"):
         def test_chat(model_client):
             response = model_client.chat.completions.create(...)
     """
+    import openai
     from infra import PARAM_MODEL
 
     marker = request.node.get_closest_marker(PARAM_MODEL)
@@ -163,10 +164,23 @@ def model_client(request: pytest.FixtureRequest, model_pool: "ModelPool"):
     model_id = marker.args[0]
 
     try:
-        return model_pool.get_client(model_id)
+        instance = model_pool.get(model_id)
     except KeyError:
         pytest.skip(f"Model {model_id} not available in model pool")
 
+    # Acquire reference to prevent eviction during test
+    instance.acquire()
+
+    client = openai.OpenAI(
+        base_url=f"{instance.base_url}/v1",
+        api_key="not-used",
+    )
+
+    yield client
+
+    # Release reference to allow eviction
+    instance.release()
+
 
 @pytest.fixture
 def model_base_url(request: pytest.FixtureRequest, model_pool: "ModelPool") -> str:
@@ -189,6 +203,14 @@ def model_base_url(request: pytest.FixtureRequest, model_pool: "ModelPool") -> s
     model_id = marker.args[0]
 
     try:
-        return model_pool.get_base_url(model_id)
+        instance = model_pool.get(model_id)
     except KeyError:
         pytest.skip(f"Model {model_id} not available in model pool")
+
+    # Acquire reference to prevent eviction during test
+    instance.acquire()
+
+    yield instance.base_url
+
+    # Release reference to allow eviction
+    instance.release()