New tests in internal/handlers, with no network: the status and JSON
error body the image route answers for a missing, wrong, unpadded,
upper-case or expired signature on a host not on the allowlist, an
unparseable path, localhost as the upstream host, and an upstream
error; that an allowlisted host is served without a signature and
another host only with a valid one; and the answers of /robots.txt and
the health check. An expired signature is answered 401, as the code
does, where the issue body expected 410. No code changes.
Model: opus-5-5
