fix: SearXNG 引擎优先级调整 — Bing 置顶，百度垫底

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/pkg/plugins/web_search/plugin.go

@@ -112,7 +112,7 @@ func (t *WebSearchTool) Execute(_ context.Context, args map[string]interface{})
 }
 
 // China-accessible SearXNG engines (baidu, sogou, 360search, bing all work from China)
-const searxngEngines = "baidu,sogou,360search,bing"
+const searxngEngines = "bing,sogou,360search,baidu"
 
 func (t *WebSearchTool) searchViaSearXNG(query string) (*sdk.ToolResult, error) {
 	apiURL := fmt.Sprintf("%s/search?format=json&engines=%s&q=%s",