fix(web_search): add diagnostic logging for Brave empty results

When the Brave Search API returns HTTP 200 with zero results, log a
warning with the response body preview to help diagnose silent failures.
This addresses cases where the API response format has changed or a
non-standard error response is misinterpreted as a successful empty
result, leaving the LLM with a misleading "No results" message.

Previously, empty results were silently returned to the LLM as "No
results for: <query>", making it impossible to distinguish between
genuinely empty results and API format mismatches or silent errors.

Closes #3125

Author: jincheng-xydt

pkg/tools/integration/web.go

@@ -350,6 +350,19 @@ func (p *BraveSearchProvider) Search(
 
 		results := searchResp.Web.Results
 		if len(results) == 0 {
+			// Log a warning when the API returned 200 but no results.
+			// This helps diagnose API format changes or silent errors
+			// where the response body does not match the expected structure.
+			bodyPreview := string(body)
+			if len(bodyPreview) > 300 {
+				bodyPreview = bodyPreview[:300]
+			}
+			logger.WarnCF("web_search", "Brave API returned empty results",
+				map[string]any{
+					"query":        query,
+					"status":       resp.StatusCode,
+					"body_preview": bodyPreview,
+				})
 			return fmt.Sprintf("No results for: %s", query), nil
 		}