fix(doubao): harden ask response parsing

[Astro-Han]

Description

Harden the Doubao ask flow so homepage and sidebar chrome are no longer misread as assistant output.

Related issue:

• [Bug]: 在 OpenCLI 中调用豆包时触发人机校验，导致工具执行失败 #909

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 🌐 New site adapter
• 📝 Documentation
• ♻️ Refactor
• 🔧 CI / build / tooling

Checklist

• I ran the checks relevant to this PR
• I updated tests or docs if needed
• I included output or screenshots when useful

Documentation (if adding/modifying an adapter)

• Added doc page under docs/adapters/ (if new adapter)
• Updated docs/adapters/index.md table (if new adapter)
• Updated sidebar in docs/.vitepress/config.mts (if new adapter)
• Updated README.md / README.zh-CN.md when command discoverability changed
• Used positional args for the command's primary subject unless a named flag is clearly better
• Normalized expected adapter failures to CliError subclasses instead of raw Error

Screenshots / Output

Relevant checks run in this worktree:

npm test
npm run test:adapter -- clis/doubao/
./node_modules/.bin/tsc --noEmit
npm run build

Built-in adapter execution check:

[
  {
    "Role": "User",
    "Text": "测试一下，只回复OK"
  },
  {
    "Role": "System",
    "Text": "No response within 15s. Doubao may still be generating."
  }
]