feat(duckduckgo): detect anti-bot challenge and surface ErrBlocked

DuckDuckGo intermittently serves a CAPTCHA modal ("Unfortunately, bots
use DuckDuckGo too...") instead of search results. The result selector
matches zero elements on that page, so callers used to get
([]Result{}, nil) — silent empty results that look like "no matches."

Detect the challenge via the BEM class .anomaly-modal__title and return
a typed ErrBlocked so callers can distinguish blocked from no-results
and react (retry, fallback to another engine, surface to user).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

sites/duckduckgo/page.go

@@ -23,9 +23,17 @@ func (s searchPage) GetResults() ([]Result, error) {
 }
 
 // extractResults parses search results from a DuckDuckGo results page.
+//
+// If the page is an anti-bot challenge ("Unfortunately, bots use DuckDuckGo
+// too...") rather than results, returns ErrBlocked so callers can distinguish
+// "blocked" from "no matches."
 func extractResults(doc extractor.Node) ([]Result, error) {
 	var res []Result
 
+	if isBlocked(doc) {
+		return nil, ErrBlocked
+	}
+
 	err := doc.ForEach(`article[id^="r1-"]`, func(n extractor.Node) error {
 		var r Result
 
@@ -68,6 +76,12 @@ func extractResults(doc extractor.Node) ([]Result, error) {
 	return res, err
 }
 
+// isBlocked reports whether the page is the DuckDuckGo anti-bot challenge
+// modal (".anomaly-modal__title") rather than a normal results page.
+func isBlocked(doc extractor.Node) bool {
+	return len(doc.Select(".anomaly-modal__title")) > 0
+}
+
 func (s searchPage) LoadMore() error {
 	return s.doc.ForEach(`button#more-results`, func(n extractor.Node) error {
 		slog.Info("clicking load more", "node", n)