* fix(acme): prefer new Baidu DNS API with legacy BCD fallback
Keep the existing BCD implementation and add fallback support for the newer Baidu DNS record API. Prefer the new API by default, then fall back to the legacy BCD API to reduce compatibility risk.
* fix(dns_baidu): route through _get/_post + restore legacy BCD auth headers
Per review: _baidu_dns_call now uses _get/_post with _H1.._H5 (no raw curl, no __HTTP_STATUS__ parsing); _baidu_bcd_post restores _H1.._H5 so the legacy BCD path sends the Authorization signature again (fixes 401).
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Co-authored-by: neil <github@neilpang.com>
