Fix websearch returning no results (DuckDuckGo anti-bot challenge)

DuckDuckGo's html.duckduckgo.com endpoint started serving an anti-bot
"anomaly" challenge page (HTTP 202, zero results) for plain GET
requests, breaking the websearch tool entirely (issue #270).

- Submit the query as a POST form (q + kl), matching how the real HTML
  page submits, which still returns standard results markup with 200.
- Use a current desktop Chrome User-Agent and explicit Accept/Content-Type.
- Make the result__a / result__snippet regexes tolerant of inline <b>
  highlight tags (non-greedy, dot-matches-newline) and strip tags from
  titles, since the live markup now wraps query terms in <b>.
- Add parses_ddg_html_results regression test.

Verified live: 'rust programming language' now returns real results.