[WIP] Update DeepSeek API to support V4 models

[Codex]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

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This section details on the original issue you should resolve

<issue_title>Update to support DeepSeekV4</issue_title>
<issue_description>## 更新说明

DeepSeek-V4
DeepSeek API 已支持 V4-Pro 与 V4-Flash，支持 OpenAI ChatCompletions 接口与 Anthropic 接口。访问新模型时，base_url 不变, model 参数需要改为 deepseek-v4-pro 或 deepseek-v4-flash。

旧有的 API 接口的两个模型名 deepseek-chat 与 deepseek-reasoner 将于三个月后（2026-07-24）停止使用。当前阶段内，这两个模型名分别指向 deepseek-v4-flash 的非思考模式与思考模式。

Code Update

修改枚举的名称，以及对应的模型名称，以适应最新的模型名称的变更 。</issue_description>

Comments on the Issue (you are @codex[agent] in this section)

@niltor > The agent encountered an error and was unable to start working on this issue: The service is temporarily unavailable. Please try again later. (Request id: 0621:2D04EC:9FA5EF:B0426B:69EB174C)

try again! @copilot</comment_new>