The platform-connected-checker invariant test requires every built-in
Platform enum member to have either a generic token path or a bespoke
entry in _PLATFORM_CONNECTED_CHECKERS. Platform.RELAY was added without
one, so test_all_builtins_have_checker_or_generic_token_path failed.
Relay dials OUT to a connector and is 'connected' once an endpoint URL
is configured (extra['relay_url'] or extra['url']); the capability
descriptor is negotiated at handshake time, so the URL is the only
config-level signal in the experimental phase. Add the checker plus a
synthetic-config case exercising its True path.
Ben
c803661cec