Get messages from multiple chats in bulk across the agency's creators (POST batch)

Returns the most recent messages for chats grouped by creator, across the creators the authenticated user's agency manages, in a single request.

This collapses the outer per-creator fan-out: instead of one batch call per managed creator (POST /creators/{creatorUserUuid}/chats/messages/batch), agencies send one request grouping chat UUIDs under each creator. Each creator group takes the same chatUuids, sinceMessageUuid and limit fields as the per-creator batch, so per-key behaviour is identical — just add the creator dimension.

Per-creator and per-chat errors are reported inside the 200 response body, so a single forbidden/missing creator or chat never collapses the whole request. A creator the agency does not manage is returned as { "error": "forbidden" | "not_found" } at the creator level.

<Note>Maximum 50 chat UUIDs in total across all creators, and 25 creators per request.</Note> <Note>Read-only: fetching messages in bulk does not mutate read state — messages are not marked read and isRead reflects each message's state at read time.</Note> <Info>Requires: Agency admin access</Info>