fix(html-parser): sanitize unclosed tags in markdown rendering

[WustLCQ]

• Add AST traversal validation for HTML tag syntax
• Implement regex filter (/<(\w+)[^>]*$/) for malformed tags
• Escape invalid nested tags instead of throwing parser errors

This resolves rendering crashes caused by unclosed tags like

<span>
  0<x<b
</span>

The sanitization preserves legitimate HTML tags while converting invalid structures to plain text.

Summary

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

Resolve #14356

Screenshots

Before	After

Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[crazywoola]

Cool

[crazywoola]

LGTM