fix(demo): check health() return value before printing Connected

client.health() returns bool, not raises — bare call ignored the result.
Now raise RuntimeError with a clear message if health check fails.

Author: polaz

demo/notebooks/03_langgraph_agent.ipynb

@@ -75,14 +75,18 @@
     "    from coordinode import CoordinodeClient\n",
     "\n",
     "    client = CoordinodeClient(COORDINODE_ADDR)\n",
-    "    client.health()\n",
+    "    if not client.health():\n",
+    "        client.close()\n",
+    "        raise RuntimeError(f\"CoordiNode at {COORDINODE_ADDR} is not serving health checks\")\n",
     "    print(f\"Connected to {COORDINODE_ADDR}\")\n",
     "elif _port_open(GRPC_PORT):\n",
     "    COORDINODE_ADDR = f\"localhost:{GRPC_PORT}\"\n",
     "    from coordinode import CoordinodeClient\n",
     "\n",
     "    client = CoordinodeClient(COORDINODE_ADDR)\n",
-    "    client.health()\n",
+    "    if not client.health():\n",
+    "        client.close()\n",
+    "        raise RuntimeError(f\"CoordiNode at {COORDINODE_ADDR} is not serving health checks\")\n",
     "    print(f\"Connected to {COORDINODE_ADDR}\")\n",
     "else:\n",
     "    # No server available — use the embedded in-process engine.\n",