feat(web): send honest nanobot UA for Keenable

Keenable is a first-party API, so identify as nanobot/<version> instead
of the shared spoofed-browser User-Agent used by scraping providers.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/tools/test_web_search_tool.py

@@ -143,7 +143,7 @@ async def test_keenable_search(monkeypatch):
     async def mock_post(self, url, **kw):
         assert "keenable" in url
         assert kw["headers"]["X-API-Key"] == "keen-key"
-        assert kw["headers"]["User-Agent"] == "nanobot-search-test"
+        assert kw["headers"]["User-Agent"].startswith("nanobot/")
         return _response(json={
             "results": [{
                 "title": "Keen",