feat: add username validation module for Zhihu

[P-r-e-m-i-u-m]

Closes #293

Summary

Adds zhihu.py username validation module for Zhihu social platform.

Changes

• Added user_scanner/user_scan/social/zhihu.py

Validation Logic

• Uses generic_validate() with a process callback
• Existing user: API returns "is_start" in response → Result.taken()
• Non-existing user: API returns "NotFoundException" → Result.available()

Manual Testing

• Existing: https://api.zhihu.com/books/people/excited-vczh/publications?offset=0&limit=5 → returns "is_start": true
• Non-existing: https://api.zhihu.com/books/people/thisdoesnotexist999xyz/publications?offset=0&limit=5 → returns "NotFoundException"

[kaifcodec]

@P-r-e-m-i-u-m I tested it and it works fine. Thank you for the contribution!

If you'd like to contribute further, feel free to look through the open issues, or add support for new sites that aren't already covered.