CE: 404 page stops self-canonicalizing (SEO)

[ericflo]

What

The 404 handler emitted <link rel=canonical> and og:url pointing to the missing URL, which Google guidance discourages (invites soft-404, can leak 404 URLs into search).

Verified live on 2026-04-16:

$ curl -s https://modelrelay.io/nonexistent-page-xyz | grep -E "canonical|og:url"
  <meta property="og:url" content="https://modelrelay.io/nonexistent-page-xyz">
  <link rel="canonical" href="https://modelrelay.io/nonexistent-page-xyz">

Fix

Pass / as the shell path so canonical + og:url point to the homepage. Adds a unit test covering the regression.

Test

cargo test -p modelrelay-cloud --lib -- not_found