fix(gateway): register relay connection checker

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.

tests/gateway/test_platform_connected_checkers.py

@@ -98,6 +98,8 @@ def test_checker_returns_true_when_configured(platform, checker, monkeypatch):
         mock_config.extra = {"app_id": "app", "app_secret": "sec"}
     elif platform == Platform.DINGTALK:
         mock_config.extra = {"client_id": "id", "client_secret": "sec"}
+    elif platform == Platform.RELAY:
+        mock_config.extra = {"relay_url": "wss://connector.example/relay"}
     else:
         pytest.skip(f"No synthetic config defined for {platform.value}")