test(sc): cover nemo-gym env handle wiring and non-vllm backend guard

Signed-off-by: Yuki Huang <yukih@nvidia.com>

Author: yuki-97

tests/unit/single_controller/test_single_controller_setup.py

@@ -306,3 +306,45 @@ def test_megatron_train_iters_not_set_when_disabled(self, patched_factories):
         setup_single_controller(mc, MagicMock(pad_token_id=0))
 
         assert "train_iters" not in mc.policy.get("megatron_cfg", {})
+
+    def test_nemo_gym_wires_env_handle(self, patched_factories):
+        """When _should_use_nemo_gym is True the nemo-gym actor is spun up and stored."""
+        mc = _make_master_config(colocated=True, backend="vllm")
+        mc.policy["generation"]["model_name"] = "test-model"
+        mc.policy["generation"]["stop_strings"] = None
+        mc.policy["generation"]["stop_token_ids"] = None
+        mc.policy["generation"]["top_k"] = None
+        patched_factories["setup_response_data"].return_value = (
+            list(range(8)),
+            None,
+        )
+        fake_gym_actor = MagicMock(name="nemo_gym_actor")
+
+        with (
+            patch.object(sc_setup_mod, "_should_use_nemo_gym", return_value=True),
+            patch.object(
+                sc_setup_mod, "spinup_nemo_gym_actor", return_value=fake_gym_actor
+            ) as mock_spinup,
+            patch.object(sc_setup_mod, "router_replay_enabled", return_value=False),
+        ):
+            actor_args = setup_single_controller(mc, MagicMock(pad_token_id=0))
+
+        mock_spinup.assert_called_once()
+        assert actor_args.env_handles["nemo_gym"] is fake_gym_actor
+
+    @pytest.mark.parametrize("backend", ["sglang", "megatron"])
+    def test_nemo_gym_rejects_non_vllm_backend(self, patched_factories, backend):
+        """SC nemo-gym wiring only supports vLLM; every other backend must raise."""
+        mc = _make_master_config(colocated=True, backend=backend)
+        patched_factories["setup_response_data"].return_value = (
+            list(range(8)),
+            None,
+        )
+
+        with (
+            patch.object(sc_setup_mod, "_should_use_nemo_gym", return_value=True),
+            patch.object(sc_setup_mod, "spinup_nemo_gym_actor") as mock_spinup,
+            pytest.raises(NotImplementedError, match="vllm"),
+        ):
+            setup_single_controller(mc, MagicMock(pad_token_id=0))
+        mock_spinup.assert_not_called()