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/constant.tsx

@@ -88,6 +88,7 @@ export enum Operator {
   TavilyExtract = 'TavilyExtract',
   UserFillUp = 'UserFillUp',
   StringTransform = 'StringTransform',
+  SearXNG = 'SearXNG',
 }
 
 export const SwitchLogicOperatorOptions = ['and', 'or'];
@@ -211,6 +212,9 @@ export const componentMenuList = [
   {
     name: Operator.Email,
   },
+  {
+    name: Operator.SearXNG,
+  },
 ];
 
 export const SwitchOperatorOptions = [
@@ -340,6 +344,22 @@ export const initialDuckValues = {
   },
 };
 
+export const initialSearXNGValues = {
+  top_n: '10',
+  searxng_url: '',
+  query: AgentGlobals.SysQuery,
+  outputs: {
+    formalized_content: {
+      value: '',
+      type: 'string',
+    },
+    json: {
+      value: [],
+      type: 'Array<Object>',
+    },
+  },
+};
+
 export const initialBaiduValues = {
   top_n: 10,
   ...initialQueryBaseValues,
@@ -807,6 +827,7 @@ export const RestrictedUpstreamMap = {
   [Operator.GitHub]: [Operator.Begin, Operator.Retrieval],
   [Operator.BaiduFanyi]: [Operator.Begin, Operator.Retrieval],
   [Operator.QWeather]: [Operator.Begin, Operator.Retrieval],
+  [Operator.SearXNG]: [Operator.Begin, Operator.Retrieval],
   [Operator.ExeSQL]: [Operator.Begin],
   [Operator.Switch]: [Operator.Begin],
   [Operator.WenCai]: [Operator.Begin],
@@ -851,6 +872,7 @@ export const NodeMap = {
   [Operator.GitHub]: 'ragNode',
   [Operator.BaiduFanyi]: 'ragNode',
   [Operator.QWeather]: 'ragNode',
+  [Operator.SearXNG]: 'ragNode',
   [Operator.ExeSQL]: 'ragNode',
   [Operator.Switch]: 'switchNode',
   [Operator.Concentrator]: 'logicNode',