fix(demo): pin coordinode SDK in 03 notebook Colab branch, surface unhealthy-port fallback

- Add _SDK_PIP_SPEC alongside _EMBEDDED_PIP_SPEC; use the pinned spec in
  the Colab branch so the SDK and embedded package resolve to the same
  git commit. Outside Colab the unpinned "coordinode" still comes from
  PyPI (editable installs / released wheels).
- Port-probe fallback path now prints an explicit WARNING before
  switching to LocalClient(":memory:") — previously the silent close()
  could surprise a user who expected the local server to be used.

Author: polaz

demo/notebooks/03_langgraph_agent.ipynb

@@ -45,6 +45,10 @@
     "    \"git+https://github.com/structured-world/coordinode-python.git\"\n",
     "    \"@c2ce32064dd7f6495f0998049c0cc2f6f7a5767d#subdirectory=coordinode-embedded\"\n",
     ")\n",
+    "_SDK_PIP_SPEC = (\n",
+    "    \"git+https://github.com/structured-world/coordinode-python.git\"\n",
+    "    \"@c2ce32064dd7f6495f0998049c0cc2f6f7a5767d#subdirectory=coordinode\"\n",
+    ")\n",
     "\n",
     "# Install coordinode-embedded in Colab only (requires Rust build).\n",
     "if IN_COLAB and not os.environ.get(\"COORDINODE_ADDR\"):\n",
@@ -95,14 +99,15 @@
     "        timeout=600,\n",
     "    )\n",
     "\n",
+    "_coordinode_spec = _SDK_PIP_SPEC if IN_COLAB else \"coordinode\"\n",
     "subprocess.run(\n",
     "    [\n",
     "        sys.executable,\n",
     "        \"-m\",\n",
     "        \"pip\",\n",
     "        \"install\",\n",
     "        \"-q\",\n",
-    "        \"coordinode\",\n",
+    "        _coordinode_spec,\n",
     "        \"langchain-community\",\n",
     "        \"langchain-openai\",\n",
     "        \"langgraph\",\n",
@@ -175,7 +180,8 @@
     "            print(f\"Connected to {COORDINODE_ADDR}\")\n",
     "            _use_embedded = False\n",
     "        else:\n",
-    "            # Port is open but not a CoordiNode server \u2014 fall through to embedded.\n",
+    "            # Port is open but server is unhealthy \u2014 surface clearly before falling back.\n",
+    "            print(f\"WARNING: port {grpc_port} is open but health check failed \u2014 falling back to embedded LocalClient(':memory:'). Data will be in-process only.\")\n",
     "            client.close()\n",
     "\n",
     "if _use_embedded:\n",