Add _YANDEX_TARPIT_MAX_RETRIES=2 constant and rewrite fetch_around with a
loop: on status_code==0 (curl exit 28, bandwidth tarpit) or captcha detected,
call _rotate_ip() + sleep(2s) and retry. Non-zero non-200 codes (404 etc.)
return [] immediately without rotation. Mirrors the existing exhaustive-path
pattern in _walk_price_range. 4 new unit tests cover: tarpit→rotate→success,
captcha→rotate→success, exhausted retries→empty, 404→no-rotate.
