fix: revise reasoning_key attribute to OpenRouter

[Soulter]

Modifications / 改动点

• This is NOT a breaking change. / 这不是一个破坏性变更。

Screenshots or Test Results / 运行截图或测试结果

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Checklist / 检查清单

• 😊 If there are new features added in the PR, I have discussed it with the authors through issues/emails, etc.
  / 如果 PR 中有新加入的功能，已经通过 Issue / 邮件等方式和作者讨论过。

• 👀 My changes have been well-tested, and "Verification Steps" and "Screenshots" have been provided above.
  / 我的更改经过了良好的测试，并已在上方提供了“验证步骤”和“运行截图”。

• 🤓 I have ensured that no new dependencies are introduced, OR if new dependencies are introduced, they have been added to the appropriate locations in requirements.txt and pyproject.toml.
  / 我确保没有引入新依赖库，或者引入了新依赖库的同时将其添加到 requirements.txt 和 pyproject.toml 文件相应位置。

• 😮 My changes do not introduce malicious code.
  / 我的更改没有引入恶意代码。

Summary by Sourcery

Bug Fixes:

• Align the OpenRouter provider's reasoning_key attribute with the expected reasoning field name.

[sourcery-ai]

Hey - I've left some high level feedback:

• Since reasoning_key = "reasoning" looks provider-specific, consider defining this as a shared constant or configuration entry (similar to other providers, if any) so that changes to OpenRouter's reasoning field name can be managed centrally rather than hardcoded here.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Since `reasoning_key = "reasoning"` looks provider-specific, consider defining this as a shared constant or configuration entry (similar to other providers, if any) so that changes to OpenRouter's reasoning field name can be managed centrally rather than hardcoded here.

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[gemini-code-assist]

Code Review

This pull request adds a reasoning_key set to "reasoning" for the OpenRouter provider. A potential logic error was identified where this change might cause the base class to overwrite reasoning content extracted from message tags with an empty string if the specific API field is missing. It is recommended to modify the base class or override the extraction logic to ensure reasoning content is not lost.