Fix admin search for PurchaseOrderLineItem (#12508)

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.

src/backend/InvenTree/order/admin.py

@@ -94,7 +94,14 @@ class PurchaseOrderLineItemAdmin(admin.ModelAdmin):
 
     list_display = ('order', 'part', 'quantity', 'reference')
 
-    search_fields = ('reference',)
+    search_fields = [
+        'part__part__name',
+        'part__part__description',
+        'part__SKU',
+        'order__reference',
+        'order__supplier__name',
+        'reference',
+    ]
 
     autocomplete_fields = ('order', 'part', 'destination')
 

src/backend/InvenTree/order/tests.py

@@ -728,3 +728,13 @@ class OrderUpdatedAtTest(TestCase):
             before = self._refresh(instance).updated_at
             line.delete()
             self.assertGreaterEqual(self._refresh(instance).updated_at, before)
+
+    def test_po_lineitem_admin_search(self):
+        """Test search fields for PurchaseOrderLineItemAdmin."""
+        from order.admin import PurchaseOrderLineItemAdmin
+
+        admin_class = PurchaseOrderLineItemAdmin
+        self.assertIn('part__part__name', admin_class.search_fields)
+        self.assertIn('part__SKU', admin_class.search_fields)
+        self.assertIn('order__reference', admin_class.search_fields)
+        self.assertIn('order__supplier__name', admin_class.search_fields)