Fix DuckDuckGo backend silently returning empty results (#2)

query_ddg_search() didn't raise when the duckduckgo_search library got
blocked/rate-limited and parsed zero results instead of throwing. This made
/serp/search_duck report {"error": null, "results": []} on failure, and
worse, made the /serp/search fallback chain (_search_one) treat DDG's empty
return as a success and never try Brave or Bing — the reason /serp/search
was returning "All backends are rate-limited." even when Bing worked fine
on its own. Now an empty DDG result raises, so both endpoints behave
correctly.


Claude-Session: https://claude.ai/code/session_012sXQyRk3EZHbYHas8fQsiS

Co-authored-by: Claude <noreply@anthropic.com>

serp.py

@@ -50,6 +50,20 @@ class BrowserUnavailableError(Exception):
         super().__init__("Playwright browser is not available (it failed to start at startup).")
 
 
+class DuckDuckGoBlockedException(Exception):
+    """Raised when DuckDuckGo returns zero results.
+
+    The duckduckgo_search library doesn't always raise when its request is
+    blocked/rate-limited — it can also just parse zero results from the
+    anti-bot response page and return an empty list. Since callers (the
+    `/serp/search` fallback chain in particular) rely on an exception to know
+    a backend failed, treat an empty result as a failure too.
+    """
+
+    def __init__(self, *args):
+        super().__init__("DuckDuckGo returned no results, likely blocked or rate-limited")
+
+
 _PLAYWRIGHT_CONCURRENCY_SEMAPHORE = Semaphore(PLAYWRIGHT_CONCURRENCY_LIMIT)
 
 
@@ -260,6 +274,9 @@ async def query_ddg_search(q: str, n_results: int = 10):
         results.append(
             {"title": result["title"], "body": result["body"], "href": result["href"]})
 
+    if not results:
+        raise DuckDuckGoBlockedException()
+
     return results