Tests use a fake HTTP client injected into Client to cover discovery
(Link headers, Atom, RSS and HTML bodies), subscribe/unsubscribe
requests, and signature verification. The workflow is adapted from
p3k-http's and runs on PHP 7.3 through 8.4, since PHPUnit 9.6 needs 7.3.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
