fix(docs): remove invalid const from await using declaration

`await using const` is not valid syntax per TC39 Explicit Resource
Management proposal. Changed to `await using session = ...`.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: JacobFV

examples/quickstart.ipynb

@@ -174,15 +174,7 @@
    "execution_count": null,
    "metadata": {},
    "outputs": [],
-   "source": [
-    "{\n",
-    "    await using const session = client.session('agi-0');\n",
-    "    const result = await session.runTask('Search for the latest Node.js LTS version');\n",
-    "    console.log(`Result: ${result.data}`);\n",
-    "    console.log(`Duration: ${result.metadata.duration}s`);\n",
-    "    console.log(`Steps: ${result.metadata.steps}`);\n",
-    "}"
-   ]
+   "source": "{\n    await using session = client.session('agi-0');\n    const result = await session.runTask('Search for the latest Node.js LTS version');\n    console.log(`Result: ${result.data}`);\n    console.log(`Duration: ${result.metadata.duration}s`);\n    console.log(`Steps: ${result.metadata.steps}`);\n}"
   }
  ],
  "metadata": {