Batch Update Messages

Apply one label change to up to 50 messages in a single request. The same add_labels and remove_labels apply to every message id, and at least one of them must be provided. The update is atomic: either all resolved messages are updated or none are. Missing or restricted ids are silently excluded; compare count against limit to detect exclusions. **CLI:** ``bash agentmail inboxes messages batch-update --inbox-id <inbox_id> --message-ids <id1> --message-ids <id2> --add-labels read --remove-labels unread