Security Advisory: Constrain and validate userscript WebSocket tasks

Security Improvement

A security improvement has been submitted via PR #483: https://github.com/JoeanAmier/XHS-Downloader/pull/483

Summary: Constrain and validate userscript WebSocket tasks

This PR was generated by an automated security scan and has passed focused tests. Please review the PR for details and merge at your discretion.

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Submitted by: @failsafesecurity (FailSafe Security Research)