releaseConnection method
Release a connection back to the pool.
Implementation
void releaseConnection(PooledConnection connection) {
_ensurePoolExists(connection.relayUrl);
final available = _availableConnections[connection.relayUrl] ?? [];
connection.markAsAvailable();
available.add(connection);
_availableConnections[connection.relayUrl] = available;
logger.log('Connection released for relay: ${connection.relayUrl}');
}