[bug] fix pptx_write tool error

[hsluoyz]

Model is Azure's gpt-5.4-pro

first time encountered error:

[
  {
    "type": "text",
    "text": "Failed to write PowerPoint file: Error: Font Awesome icon not found: domino\n    at Object.iconSvgData (file:///D:/github_repos/openagent/tool/pptx-worker/worker.mjs:80:11)\n    at file:///C:/Users/luo/AppData/Local/Temp/openagent-pptx-build-4172819478.mjs:587:19\n    at Array.forEach (<anonymous>)\n    at build (file:///C:/Users/luo/AppData/Local/Temp/openagent-pptx-build-4172819478.mjs:573:14)\n    at generateDeck (file:///D:/github_repos/openagent/tool/pptx-worker/worker.mjs:151:9)"
  }
]

second time succeeds