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, or a
valid one sent for its parent domain, a sibling host, a subdomain or
the host with another domain appended; 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 and README.md say, where the
issue body expected 410. No code changes.
Model: opus-5-5
