fix(safe-outputs): preserve custom XML tags, only escape known HTML/SVG elements

[Copilot]

The safe-outputs sanitizer was converting all unrecognized tags to parentheses — including custom XML like MSBuild's <AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>, which would be mangled to (AppendTargetFrameworkToOutputPath)false(/AppendTargetFrameworkToOutputPath) even inside markdown code fences.

Changes

• sanitize_content_core.cjs: Changed convertXmlTags() from an allowlist model (preserve GFM-safe tags, convert everything else) to a denylist model (only convert known HTML5 + SVG + MathML elements). Custom XML tags — anything not in the standard web vocabulary — pass through unchanged.

  • HTML5 elements: full element set added
  • SVG elements: included since they're valid inline HTML5 and support XSS vectors via event handlers (onload, etc.)
  • MathML: math root element included

• sanitize_content.test.cjs: Added tests covering custom XML preservation and confirming known HTML elements are still converted.

Behavior

<!-- Before: mangled even inside code fences -->
(AppendTargetFrameworkToOutputPath)false(/AppendTargetFrameworkToOutputPath)

<!-- After: preserved -->
<AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>

Known injection vectors are unchanged — <div>, <script>, <svg onload="..."> etc. are still converted to parentheses.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Safe-outputs add-comment strips < and > from markdown code blocks</issue_title>
<issue_description>The safe-outputs sanitizer replaces < and > with ( and ) in the posted comment body, even inside markdown code fences. This breaks XML/HTML code examples — critical for workflows that analyze MSBuild/XML-based build systems.

Example

The agent produces:

<AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>

But the posted comment contains:

(AppendTargetFrameworkToOutputPath)false(/AppendTargetFrameworkToOutputPath)

Repro

ViktorHofer/msbuild-s1#4 (comment)

Workflow: MSBuild Build Failure Analyzer run</issue_description>

Comments on the Issue (you are @copilot in this section)

@pelikhan @copilot update JavaScript xml escaped to only escape known HTML elements.

• Fixes Safe-outputs add-comment strips < and > from markdown code blocks #18310

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