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/pages/agent/form-sheet/form-config-map.tsx

@@ -27,6 +27,7 @@ import QWeatherForm from '../form/qweather-form';
 import RelevantForm from '../form/relevant-form';
 import RetrievalForm from '../form/retrieval-form/next';
 import RewriteQuestionForm from '../form/rewrite-question-form';
+import SearXNGForm from '../form/searxng-form';
 import StringTransformForm from '../form/string-transform-form';
 import SwitchForm from '../form/switch-form';
 import TavilyExtractForm from '../form/tavily-extract-form';
@@ -132,6 +133,9 @@ export const FormConfigMap = {
   [Operator.Invoke]: {
     component: InvokeForm,
   },
+  [Operator.SearXNG]: {
+    component: SearXNGForm,
+  },
   [Operator.Concentrator]: {
     component: () => <></>,
   },