The DuckDuckGo HTML fallback returns redirect URLs (//duckduckgo.com/l/?uddg=...)
instead of actual page URLs. This caused fetch_webpage_content() to reject them
instantly because _public_http_url() requires an http/https scheme, making search
results unfetchable in deep research mode.
Added _resolve_url() to:
- Convert protocol-relative URLs to absolute (https:)
- Convert path-relative URLs to absolute
- Extract the real URL from DuckDuckGo's /l/?uddg= redirect parameters
