Check whether a domain name is registered, using real-time RDAP data
(the IETF-standardized successor to WHOIS). Use for "is domain-x.com
available/registered", "who is domain-x.com registered with", or "when
does domain-x.com expire" style questions.
`domain` is a bare domain like "example.com" -- no protocol prefix, no
path, no spaces.
If the domain is NOT registered, returns `{"domain": ..., "registered":
false}` -- this is a normal, meaningful answer (the domain is
available), not an error.
If the domain IS registered, returns `{"domain", "registered": true,
"status", "registered_date", "expiration_date", "last_changed_date",
"registrar"}`. `status` is a list of RDAP status codes (e.g. "active",
"client transfer prohibited"). Dates are ISO 8601. `registrar` is the
registrar's name, or null if the registry redacts it for privacy.
For obviously malformed input (not a domain -- e.g. missing a dot, has
spaces, or includes a protocol like "https://"), returns a clean
`{"error": ...}` instead of calling the API.