* fix(gateway): serve XML artifacts as attachments to block same-origin script
GET /api/threads/{id}/artifacts/{path} forced only text/html,
application/xhtml+xml and image/svg+xml to download. Every other XML
document was served inline from the application origin: `.xml` guesses
to text/xml or application/xml depending on the host's mime.types, and
both fell through to the inline text branches. Browsers render any XML
MIME type as a document and run an XHTML-namespaced <script> inside it,
so a report.xml written by a prompt-injected agent and opened from a
chat link executed with the viewer's session: the HttpOnly access_token
rides same-origin fetches, and the double-submit csrf_token cookie is
JS-readable, so state-changing calls are reachable as well.
Treat HTML plus every WHATWG XML MIME type (text/xml, application/xml,
any +xml subtype) and text/xsl, which Blink also renders as XML, as
active content. A single helper owns the rule for both the regular-file
and the .skill-archive-member branches. The artifacts panel already
previews .xml as code through a ranged fetch, so preview and editing
keep working against the attachment response.
* docs(frontend): name XML among the artifacts the Gateway downloads
Review follow-up on #5353: resolveArtifactOpenURL's comment still named
only HTML/SVG as the active content the Gateway serves as a download.
XML documents now join that bucket, so the frontend note matches the
Gateway rule. Comment-only; no behavior change.
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