Function isCompactBoundaryMessage

src/utils/messages.ts:4647–4651

(
  message: Message | NormalizedMessage,
)

 * Checks if a message is a compact boundary marker
 */
export function isCompactBoundaryMessage(
  message: Message | NormalizedMessage,
): message is SystemCompactBoundaryMessage {
  return message?.type === 'system' && message.subtype === 'compact_boundary'
}

/**
 * Finds the index of the last compact boundary marker in the messages array