websearch: SearXNG 默认只查 bing,360search

默认 general 类别在 Docker 任务容器里要 20.0 秒才回
（wikipedia/wikidata/brave/duckduckgo/google 引擎全超时，
只有 bing+360search 回），撞 MCP 默认 20 秒超时；随后
DuckDuckGo 经代理也超时，整条链失败。代理本身无问题
（Tinyproxy 白名单早已放行 searx 与 duckduckgo）。
修法：_provider_searxng 默认 engines=bing,360search，
SEARXNG_ENGINES 可覆盖、空值恢复旧行为；worker 搜索
env 透传补 SEARXNG_ENGINES。容器实测：钉住后 0.26 秒
回 3 条（首条北京天气），auto 链 0.15 秒成功；旧 20.0 秒。
测试：新增 test_websearch_searxng 3 项，MCP/worker 共
21 项全过。生产未部署。

cloud/tests/test_websearch_searxng.py

@@ -0,0 +1,52 @@
+"""SearXNG provider: engines are pinned, not left to the default
+general category (2026-10-30: default took 20s in a Docker task,
+pinned bing,360search took 0.47s via the same egress proxy)."""
+
+import json
+import os
+import sys
+import unittest
+
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", ".."))
+from cloud.websearch import _provider_searxng  # noqa: E402
+
+
+class _FakeHttp:
+    def __init__(self):
+        self.urls = []
+
+    def get(self, url, headers=None):
+        self.urls.append(url)
+        body = json.dumps({"results": [
+            {"title": "Beijing weather", "url": "https://example.test/w",
+             "content": "sunny"}]}).encode("utf-8")
+        return 200, body, "application/json"
+
+
+class TestSearXNGEngines(unittest.TestCase):
+    def test_default_pins_bing_360search(self):
+        http = _FakeHttp()
+        results, _ = _provider_searxng(
+            "Beijing weather", 3, {"SEARXNG_URL": "http://searx.test"},
+            http)
+        self.assertEqual(len(results), 1)
+        self.assertIn("engines=bing%2C360search", http.urls[0])
+
+    def test_env_overrides_pin(self):
+        http = _FakeHttp()
+        _provider_searxng(
+            "q", 3, {"SEARXNG_URL": "http://searx.test",
+                     "SEARXNG_ENGINES": "bing"}, http)
+        self.assertIn("engines=bing", http.urls[0])
+        self.assertNotIn("360search", http.urls[0])
+
+    def test_empty_env_restores_unpinned(self):
+        http = _FakeHttp()
+        _provider_searxng(
+            "q", 3, {"SEARXNG_URL": "http://searx.test",
+                     "SEARXNG_ENGINES": ""}, http)
+        self.assertNotIn("engines=", http.urls[0])
+
+
+if __name__ == "__main__":
+    unittest.main()