🐛 fix: fix fallback behavior of default mode in AgentRuntime

[hezhijie0327]

💻 变更类型 | Change Type

• ✨ feat
• 🐛 fix
• ♻️ refactor
• 💄 style
• 👷 build
• ⚡️ perf
• 📝 docs
• 🔨 chore

🔀 变更说明 | Description of Change

触发原因：
当时 Fallback 判定写的有问题，不应该用 APIKey 值是否存在作为判定依据，导致只要模型的 APIKey 环境变量为空，就走 OpenAI。也反向印证了昨天 Qwen 行为在设置 QWEN_API_KEY 后就正常

改进方法：
改为判定 *_API_KEY 是否在 llmConfig 中声明

close #4809

📝 补充信息 | Additional Information

测试场景：

环境变量	设置值
OPENAI_PROXY_URL	https://proxy.example.org/v1
ANTHROPIC_API_KEY	空

Before	After

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👍 @hezhijie0327

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.53%. Comparing base (3da1704) to head (a68c7b5).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##             main    #4813     +/-   ##
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  Coverage   91.53%   91.53%             
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  Files         538      538             
  Lines       37803    37803             
  Branches     3651     2478   -1173     
=========================================
  Hits        34603    34603             
  Misses       3200     3200             

Flag	Coverage Δ
app	91.53% <100.00%> (ø)
server	96.21% <ø> (ø)

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Name	Status	Preview	Comments	Updated (UTC)
lobe-chat-preview	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Nov 27, 2024 4:04pm

[lobehubbot]

❤️ Great PR @hezhijie0327 ❤️

The growth of project is inseparable from user feedback and contribution, thanks for your contribution! If you are interesting with the lobehub developer community, please join our discord and then dm @arvinxx or @canisminor1990. They will invite you to our private developer channel. We are talking about the lobe-chat development or sharing ai newsletter around the world.
项目的成长离不开用户反馈和贡献，感谢您的贡献! 如果您对 LobeHub 开发者社区感兴趣，请加入我们的 discord，然后私信 @arvinxx 或 @canisminor1990。他们会邀请您加入我们的私密开发者频道。我们将会讨论关于 Lobe Chat 的开发，分享和讨论全球范围内的 AI 消息。

[lobehubbot]

🎉 This PR is included in version 1.34.3 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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