fix: 修复 web_search 搜索超时和噪音问题

- 移除baidu/360search引擎（安全验证拦截），保留bing+sogou
- 超时从10s增加到25s，SearXNG内部timeout=20s
- 添加User-Agent
- 跳过失败的工具结果推送，避免"百度被安全验证挡住"等噪音刷屏

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

backend/ai-core/internal/tools/web_search.go

@@ -11,7 +11,7 @@ import (
 	"time"
 )
 
-// WebSearchTool 网页搜索工具 - 基于 SearXNG (或 DuckDuckGo fallback)
+// WebSearchTool 网页搜索工具 - 基于 SearXNG
 type WebSearchTool struct {
 	client     *http.Client
 	timeout    time.Duration
@@ -22,9 +22,9 @@ type WebSearchTool struct {
 func NewWebSearchTool() *WebSearchTool {
 	return &WebSearchTool{
 		client: &http.Client{
-			Timeout: 10 * time.Second,
+			Timeout: 25 * time.Second,
 		},
-		timeout: 10 * time.Second,
+		timeout: 25 * time.Second,
 	}
 }
 
@@ -32,9 +32,9 @@ func NewWebSearchTool() *WebSearchTool {
 func NewWebSearchToolWithURL(searxngURL string) *WebSearchTool {
 	return &WebSearchTool{
 		client: &http.Client{
-			Timeout: 10 * time.Second,
+			Timeout: 25 * time.Second,
 		},
-		timeout:    10 * time.Second,
+		timeout:    25 * time.Second,
 		searxngURL: strings.TrimRight(searxngURL, "/"),
 	}
 }
@@ -97,13 +97,16 @@ func (t *WebSearchTool) Execute(ctx context.Context, arguments map[string]interf
 }
 
 func (t *WebSearchTool) searchViaSearXNG(ctx context.Context, query string) (*ToolResult, error) {
-	apiURL := fmt.Sprintf("%s/search?format=json&engines=bing,sogou,360search,baidu&q=%s",
+	// 只用 bing+sogou，去掉 baidu（安全验证拦截）和360search（不稳定）
+	// timeout=20 让 SearXNG 内部限速，避免慢引擎拖垮整次搜索
+	apiURL := fmt.Sprintf("%s/search?format=json&engines=bing,sogou&timeout=20&q=%s",
 		t.searxngURL, url.QueryEscape(query))
 
 	req, err := http.NewRequestWithContext(ctx, "GET", apiURL, nil)
 	if err != nil {
 		return &ToolResult{ToolName: "web_search", Success: false, Error: fmt.Sprintf("创建请求失败: %v", err)}, nil
 	}
+	req.Header.Set("User-Agent", "Mozilla/5.0 (compatible; CyreneBot/1.0)")
 
 	resp, err := t.client.Do(req)
 	if err != nil {