Feat: add SearXNG search tool to Agent (frontend + backend, i18n) (#9699)

### What problem does this PR solve?

This PR integrates SearXNG as a new search tool for Agents. It adds
corresponding form/config UI on the frontend and a new tool
implementation on the backend, enabling aggregated web searches via a
self-hosted SearXNG instance within chats/workflows. It also adds
multilingual copy to support internationalized presentation and
configuration guidance.

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

### What’s Changed
- Frontend: new SearXNG tool configuration, forms, and command wiring
  - Main changes under `web/src/pages/agent/`
- New components and form entries are connected to Agent tool selection
and workflow node configuration
- Backend: new tool implementation
- `agent/tools/searxng.py`: connects to a SearXNG instance and performs
search based on the provided instance URL and query parameters
- i18n updates
- Added/updated keys under `web/src/locales/`: `searXNG` and
`searXNGDescription`
- English reference in
[web/src/locales/en.ts](cci:7://file:///c:/Users/ruy_x/Work/CRSC/2025/Software_Development/2025.8/ragflow-pr/ragflow/web/src/locales/en.ts:0:0-0:0):
    - `searXNG: 'SearXNG'`
- `searXNGDescription: 'A component that searches via your provided
SearXNG instance URL. Specify TopN and the instance URL.'`
- Other languages have `searXNG` and `searXNGDescription` added as well,
but accuracy is only guaranteed for English, Simplified Chinese, and
Traditional Chinese.

---------

Co-authored-by: xurui <xurui@crscd.com.cn>

web/src/locales/zh.ts

@@ -971,6 +971,9 @@ General：实体和关系提取提示来自 GitHub - microsoft/graphrag：基于
       duckDuckGo: 'DuckDuckGo',
       duckDuckGoDescription:
         '此元件用於從 www.duckduckgo.com 取得搜尋結果。通常，它作為知識庫的補充。 Top N 指定您需要調整的搜尋結果數。',
+      searXNG: 'SearXNG',
+      searXNGDescription:
+        '该组件通过您提供的 SearXNG 实例地址进行搜索。请设置 Top N 和实例 URL。',
       channel: '频道',
       channelTip: '针对该组件的输入进行文本搜索或新闻搜索',
       text: '文本',