Adds displayname to existing assertions and new tests covering:
null/short/invalid-charset displayname, duplicate displayname,
findability by displayname. AuthenticateUser tests pick up the
seedUser displayname argument.
9 cases: null/empty/malformed email -> BadRequest; null/empty
password -> BadRequest; unknown email -> Unauthorized; wrong
password -> Unauthorized; valid creds return the User entity;
isAdmin flag survives the auth round-trip. fails red - the
AuthenticateUser class does not exist yet.