Artifact public sharing fails with "unscannable" content scan error

## Description
Attempting to make a Claude Code Artifact public (via the share menu) fails
with the following error, even after republishing a new version of the
artifact to force a fresh scan:

{
"error": "frame: public serving requires the served version's content scan to be dispatched (unscannable)",
"reason": "unscannable"
}

## Steps to reproduce
1. Publish an HTML artifact via the Artifact tool in Claude Code.
2. Open the artifact's share menu and attempt to switch visibility to "Public".
3. The toggle fails with the "unscannable" error above.
4. Republishing the same artifact (new version, same URL) does not resolve it —
the error persists.

## Affected artifacts
Reproduced consistently across 6 different self-contained HTML artifacts
published in the same session. Files are static HTML (single file, inline
CSS/JS, embedded font as a base64 data: URI), largest is ~147KB — well
under the documented 16MB artifact size cap, so this does not appear to be
a size-related issue. Happy to share specific artifact URLs privately if
useful for debugging.

## Environment
- Claude Code CLI on Windows (win32)
- Issue observed 2026-08-18

Start with the Claude Code Artifact public-sharing flow and the content-scan dispatch path; reproduce the failure using a self-contained HTML artifact with inline CSS/JS and an embedded data URI. Done means the share-menu visibility toggle succeeds for these artifacts without the "unscannable" error after republishing.

索引モデルが issue の本文から書いたものです。