fix(zhihu): decode numeric entities in answer detail

[lenovobenben]

Summary

• decode decimal and hexadecimal HTML numeric entities in Zhihu answer-detail content
• keep invalid numeric entities unchanged instead of throwing
• add unit coverage for numeric entity decoding

Why

Zhihu answer HTML can include numeric entities such as " and &. The answer-detail stripper decoded a few named entities but left numeric entities visible in the returned text.

Validation

• npx vitest run --project adapter clis/zhihu/answer-detail.test.js clis/zhihu/question.test.js
• npm run dev -- zhihu answer-detail "https://www.zhihu.com/question/2033593594901160569/answer/2034724552681453448" --max-content 500 -f json
• git diff --check