fix(notebooks): repair JSON source format and harden notebooks

- Repair character-by-character cell source serialization in
  01_llama (cell 6) and 02_langchain (cells 6, 18) — source must be
  a list of full lines, not single-char list elements
- Delay COORDINODE_PORT int() parsing until the else-branch so a
  malformed port value does not crash startup when COORDINODE_ADDR is set
- Replace two-step DELETE+DELETE with DETACH DELETE in 00_seed_data
  cleanup to avoid duplicate-edge errors from undirected MATCH -[r]-()
- Cap explicit LIMIT to min(n, 20) in 03_langgraph query_facts instead
  of unconditionally replacing it, preserving LIMIT 1 and similar
- Wrap create_text_index calls inside try-blocks with idx_created flag
  in three FTS integration tests so node cleanup always runs even when
  index creation fails

Author: polaz

demo/notebooks/00_seed_data.ipynb

@@ -119,7 +119,20 @@
    "id": "a1b2c3d4-0000-0000-0000-000000000007",
    "metadata": {},
    "outputs": [],
-   "source": "import uuid\n\nDEMO_TAG = os.environ.get(\"COORDINODE_DEMO_TAG\") or f\"seed_data_{uuid.uuid4().hex[:8]}\"\nprint(\"Using DEMO_TAG:\", DEMO_TAG)\n# Delete relationships first, then nodes — two-step ensures correct cleanup across\n# all CoordiNode versions (embedded and server).\nclient.cypher(\n    \"MATCH (n {demo: true, demo_tag: $tag})-[r]-() DELETE r\",\n    params={\"tag\": DEMO_TAG},\n)\nclient.cypher(\n    \"MATCH (n {demo: true, demo_tag: $tag}) DELETE n\",\n    params={\"tag\": DEMO_TAG},\n)\nprint(\"Previous demo data removed\")"
+   "source": [
+    "import uuid\n",
+    "\n",
+    "DEMO_TAG = os.environ.get(\"COORDINODE_DEMO_TAG\") or f\"seed_data_{uuid.uuid4().hex[:8]}\"\n",
+    "print(\"Using DEMO_TAG:\", DEMO_TAG)\n",
+    "# Remove prior demo nodes and any attached relationships in one step to avoid\n",
+    "# duplicate relationship matches during cleanup (undirected MATCH -[r]-() returns\n",
+    "# each edge twice — once per endpoint — causing duplicate-delete errors).\n",
+    "client.cypher(\n",
+    "    \"MATCH (n {demo: true, demo_tag: $tag}) DETACH DELETE n\",\n",
+    "    params={\"tag\": DEMO_TAG},\n",
+    ")\n",
+    "print(\"Previous demo data removed\")"
+   ]
   },
   {
    "cell_type": "markdown",
@@ -373,4 +386,4 @@
  },
  "nbformat": 4,
  "nbformat_minor": 5
-}
+}