Create multiple manifest shipments in a single request, optionally
with their nested line items.

Limits & semantics

• Maximum 30 shipments per request. Requests exceeding this limit
  are rejected with 422 and an error in the manifest_shipments field.
• All-or-nothing. The entire batch runs inside a database transaction;
  if any shipment fails validation (or a unique-constraint check),
  no shipments are persisted and the response lists every offending
  entry by its array index.
• Duplicate detection within the batch. Tracking numbers are
  compared case-insensitively (matching the database collation). If the
  same tracking number appears twice in one request, every offending
  entry is returned with Tracking number is duplicated within this batch.
• Per-account uniqueness. Beyond intra-batch detection, each shipment
  is also validated against existing rows for the authenticated account;
  a collision returns Tracking number has already been taken.

Request shape

Pass a JSON object with a top-level manifest_shipments array. Each
element supports the same fields as a single POST /api/v1/manifest/shipments,
including the nested shipment_items_attributes array. Required per
item: tracking_number, carrier. The server assigns the id; do
not supply one.

Response shape

• 201 Created — { manifest_shipments: [<full shipment object>, ...] }.
  Each entry mirrors the shape returned by the single-create endpoint
  and includes the server-assigned id plus all nested items.
• 422 Unprocessable Entity — { errors: [{ index, tracking_number, messages }, ...] },
  one entry per offending shipment. index is the position in the
  request array; tracking_number is the value supplied (or null);
  messages is the list of validation errors as strings.