Function validate_session_control_id 

pub fn validate_session_control_id(
    field: &'static str,
    value: &str,
) -> Result<(), ProviderEventValidationError>

Bounds check for the mode_id/option_id/model_id an ACP session control command (ControlCommand::SetSessionMode/ SetSessionConfigOption/SetSessionModel) carries – the same bound as any other provider-scoped id (PROVIDER_EVENT_ID_MAX_BYTES), required and free of control characters. Mirrors what gate4agent-node-protocol’s wire boundary already enforces (deserialize_acp_control_id) before a command ever reaches gate4agent-engine; re-checked here because a ControlCommand is constructible directly (tests, other embedders), not only through that one wire.