Add Fulfillment Line Item Bin Setting Mutation

The unstable API introduces a mutation fulfillmentLineItemBinsSet that lets a store set the specific bins from which each line item of a successful fulfillment was picked. The mutation accepts a list of FulfillmentLineItemBinInput objects, each specifying a fulfillment line item ID, quantity and optional bin ID (defaults to the location’s default bin). The response returns the updated Fulfillment with new binQuantities and a list of FulfillmentLineItemBinsSetUserError when validation fails. Errors cover missing preview, non‑existent fulfillment, incomplete or invalid bin data, inventory claim errors and locking conflicts. This operation affects only inventory adjustments between bins; the fulfillment’s quantity and commitment remain unchanged. The mutation is gated to development stores with the Physical Inventory feature preview. Additionally, the Metafield definition field now returns null when a metafield’s value fails validation against its definition, and invalid metafields can be queried via the definition’s metafields connection with validationStatus: INVALID.