webui: add rehype plugin to restore HTML in Markdown table cells (#17477)

webui: add rehype plugin to restore HTML in Markdown table cells (#17477)

* webui: add rehype plugin to restore HTML in Markdown table cells

The remark/rehype pipeline neutralizes inline HTML as literal text
(remarkLiteralHtml) so that XML/HTML snippets in LLM responses display
as-is instead of being rendered. This causes <br> and <ul> markup in
table cells to show as plain text.

This plugin traverses the HAST post-conversion, parses whitelisted HTML
patterns (<br>, <ul><li>) from text nodes, and replaces them with actual
HAST element nodes. For lists, adjacent siblings must be combined first
as the AST fragmentation breaks pattern matching.

Strict validation rejects malformed markup, keeping it as raw text.

* chore: update webui build output