fix: localize LLM request errors in ChatUI

[wcqqq1214]

ChatUI LLM failure messages are hardcoded in Chinese, regardless of the selected UI language.

Modifications / 改动点

• Pass the current UI locale with chat, regenerate, and thread requests.

• Generate built-in LLM error messages in zh-CN, en-US, ru-RU, or ja-JP; default to Chinese when the locale is absent or unsupported.

• Keep existing history, normal/custom replies, and error handling unchanged. No display-time text matching or added tests.

• 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

Verification steps: with no model configured, select English/Russian/Japanese and send a new message. Its error should use that language; changing the language afterward should leave saved messages unchanged.

Checks: Ruff format/check and dashboard typecheck/build passed; API client and public OpenAPI docs regenerated; all 4 existing OpenAPI artifact checks passed.

Before — Chinese error in English UI

After — English

After — Russian

After — Japanese

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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 checked the affected WebUI instructions and screenshots in docs/zh and docs/en against the changed navigation, page structure, and labels, and updated them in this PR (or explained why no documentation update is needed). For renamed, moved, or merged entry points, I included an old entry → new entry mapping in the documentation and changelog.
  / 我已对照变化后的 WebUI 入口、页面结构和术语，核对并在本 PR 中更新 docs/zh 和 docs/en 的相关操作说明与截图（或说明无需更新文档的原因）。入口改名、移动或合并时，已在文档和 changelog 中补充 旧入口 → 新入口 对照。

• 🤓 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

Localize ChatUI LLM errors by carrying the active UI language through requests and formatting failures in the matching supported language.

Bug Fixes:

• Localize built-in LLM request failure messages according to the WebUI locale, with Chinese fallback for missing or unsupported locales.

Enhancements:

• Propagate the selected UI locale through chat, thread, and regeneration requests while preserving saved message text and existing error handling.

Documentation:

• Update the OpenAPI specification and generated artifacts to document the optional locale request field.

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

Sourcery assessment

Approved.

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