agentLoadModeValidationError function
Validates one agent.mode value (issue #680): null when value is a
legal agentLoadModeLabels label, else the error text. The single
source shared by the boot parser (cli_config.dart) and the settings
validator (config_service.dart) — both throw it as a
ConfigException, so the rule cannot drift between the two parsers.
Implementation
String? agentLoadModeValidationError(Object? value) {
if (value is String && agentLoadModeLabels.contains(value)) return null;
return '"agent.mode" must be one of: ${agentLoadModeLabels.join(', ')}';
}