Bug 30144: (QA follow-up) Exclude servicing_instruction from order API representation

The servicing_instruction column was not mapped in
Koha::Acquisition::Order->to_api_mapping, so it was exposed verbatim in
the REST API order representation. Since api/v1/swagger/definitions/order.yaml
does not declare this property and sets additionalProperties: false, any
request touching the acquisitions/orders endpoint (e.g. the "Receive
shipments" page) failed with a 500 error: "Properties not allowed:
servicing_instruction".

Exclude it from the API representation, consistent with the other
EDIFACT-only columns (line_item_id, suppliers_reference_number, etc.)
that are already excluded the same way.

Test plan:
1) Go to a vendor page, create a basket, add an order line, and close
   the basket.
2) From the vendor page, click "Receive shipments".
3) Without this patch, the orders table fails to load with a 500 error
   mentioning "Properties not allowed: servicing_instruction".
4) With this patch applied, the orders table loads correctly.
5) prove t/db_dependent/api/v1/acquisitions_orders.t

Signed-off-by: Pedro Amorim <pedro.amorim@openfifth.co.uk>

Koha/Acquisition/Order.pm

@@ -654,6 +654,7 @@ sub to_api_mapping {
         parent_ordernumber            => 'parent_order_id',
         quantityreceived              => 'quantity_received',
         replacementprice              => 'replacement_price',
+        servicing_instruction         => undef,                       # EDIFACT related
         sort1                         => 'statistics_1',
         sort1_authcat                 => 'statistics_1_authcat',
         sort2                         => 'statistics_2',