chore(deps): update pnpm to v9.15.1

Author: renovate[bot]

package.json

@@ -4,7 +4,7 @@
   "version": "0.0.3",
   "description": "stainless api monorepo",
   "author": "[email protected]",
-  "packageManager": "pnpm@9.9.0",
+  "packageManager": "pnpm@9.15.1",
   "repository": {
     "type": "git",
     "url": "https://github.com/stainless-api/stl-api.git"