fix(llms): remove tool_choice for deepseek models

[gaomeng1900]

What

Remove tool_choice from DeepSeek request bodies because DeepSeek models do not accept that parameter.

Closes #475

Type

• Breaking change
• Bug fix
• Feature / Improvement
• Refactor / Chores
• Documentation / Website / Demo / Testing

Testing

• Tested in modern browsers
• No console errors
• Types/doc added

Automated validation run:

• npx eslint packages/llms/src/utils.ts passed
• npm run build --workspace=@page-agent/llms passed
• npm run typecheck passed

Requirements / 要求

• I have read and follow the Code of Conduct and Contributing Guide . / 我已阅读并遵守行为准则。
• This PR is NOT generated by a bot or AI agent acting autonomously. I have authored or meaningfully reviewed every change. / 此 PR 不是由 bot 或 AI 自主生成的，我已亲自编写或充分审查了每一处变更。