#129 - Partially return void a restaurant order 8.6

[mariananton18]

{Summary of the changes}
Forward recType=7 to myDATA for partial cancellation of restaurant order lines (8.6).
When a restaurant order (type 8.6) contains individual charge items flagged as returned
(ChargeItemCaseFlags.Refund / 0x0002), the middleware now correctly sets recType=7 on
those lines and guarantees all amounts are positive, as required by myDATA.
Previously the recType field was never set, causing myDATA to treat returned
lines as normal order lines.

{Detail}
I added the per-item return detection in AADEFactory.cs -> GetInvoiceDetails(),
where isPartialReturnItem is evaluated as InvoiceType.Item86 && chargeItem.IsRefund().
When true, recType=7 and recTypeSpecified=true are set on the InvoiceRowType,
and Math.Abs() is applied to quantity, vatAmount and netValue to guarantee
positive values regardless of the sign the POS sends
(e.g. VIVA sends negative amounts with the refund flag).

I fixed AADEMappings.cs -> GetIncomeClassificationType() to also apply Math.Abs()
on netAmount for partial return lines in 8.6 orders.
Without this, incomeClassification.amount in the generated XML was negative,
which caused myDATA to reject the payload.

I added the PartiallyCancelInvoiceValidationTests unit test class with three scenarios:

• Scenario 1: MapToInvoicesDoc_Order86_SingleReturnedLine_SetsRecType7WithPositiveAmounts
  A single returned item with positive payload. The POS sends the item with
  ChargeItemCaseFlags.Refund and positive amounts.
  Asserts recType=7, positive netValue, vatAmount, quantity,
  positive incomeClassification.amount, and correct invoice header.

• Scenario 2: MapToInvoicesDoc_Order86_MixedLines_OnlyReturnedLineHasRecType7
  A mixed order with one normal line and one returned line.
  Asserts that only the flagged line gets recType=7,
  the normal line has no recType, and all amounts on the returned
  line are positive including incomeClassification.amount.

• Scenario 3: MapToInvoicesDoc_Order86_ReturnedLineWithNegativeAmounts_OutputsPositiveAmounts
  A returned item where the POS sends negative amounts (e.g. VIVA-style).
  Asserts that Math.Abs() normalises all values to positive
  as required by myDATA for recType=7 lines.

The existing full-void path (ReceiptCaseFlags.Void -> GetInvoiceDetailsForVoid)
and all tests in CancelInvoiceValidationTests are untouched.

Fixes [#129](Partially return/void a restaturnat order 8.6 · Issue #129 · fiskaltrust/market-gr)