When a page fails to load in a T3 browser tab, preview_status and
preview_snapshot show only chrome-error://chromewebdata/, so agents cannot
tell why. Both now include loadError with the failed URL, the browser's
error code, and its description while the tab shows the error page.

Server-rendered tabs record the failure from Playwright's requestfailed on
main-frame navigations and clear it when the next navigation starts.
Desktop-rendered tabs can fail their first load before the server connects,
so the desktop forwards did-fail-load for server tabs over the desktop
browser channel as a loadFailed event, including failures from before the
webview registered with its tab. The server uses the desktop's report only
until it observes a navigation itself.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>