Admit onboard camera preview only after its acquisition producer is ready

tests/test_xgrids_camera_gateway.py

@@ -1984,3 +1984,18 @@ def test_service_publishes_two_dynamic_camera_rows_and_stale_stop_is_safe(
             )
     finally:
         service.close()
+
+
+def test_onboard_preview_never_spawns_a_camera_producer(tmp_path, monkeypatch):
+    gateway = _gateway(tmp_path, monkeypatch)
+    try:
+        selected = gateway.select("sensor.camera.right", "192.168.1.20")
+        monkeypatch.setattr(
+            gateway, "_spawn_selected_producer",
+            lambda: pytest.fail("an onboard preview must not own camera activation"),
+        )
+        with pytest.raises(RuntimeError, match="producer is not ready"):
+            gateway.open_delivery(selected["generation"], require_recording=True)
+        assert gateway.snapshot()["recording"]["producer_alive"] is False
+    finally:
+        gateway.close()