Expand search_fields on PurchaseOrderLineItemAdmin to allow
searching by part name, part description, supplier SKU,
order reference, and supplier name.
Previously, only the optional 'reference' field was searchable,
making it impossible to find line items by part or order details.
- part__part__name: search by underlying Part name
- part__part__description: search by Part description
- part__SKU: search by Supplier SKU
- order__reference: search by Purchase Order reference
- order__supplier__name: search by Supplier name
Also adds a unit test to verify the search_fields are correctly
configured.
Bhumin Paladiya