DEV-5582 Call TaxCloud Returned on canceled unpaid orders

[azane-code]

DEV-5582: Reverse TaxCloud sale when order is canceled (unpaid, no invoices)

Summary

When an order is canceled and has no invoices, we now call the TaxCloud Returned API so the sale is reversed in TaxCloud. Whether we call Returned is based on TaxCloud’s OrderDetails API (non-empty CapturedDate). This uses TaxCloud v1 only (no Void).

Changes

• Observer/Sales/Cancel.php (new): Listens to order_cancel_after and sales_order_save_after; runs only when the order is canceled, has no invoices, and TaxCloud reports it as captured (OrderDetails returns a non-empty CapturedDate). Calls Api::getOrderDetails($order) then Api::returnOrderCancellation($order);
• Model/Api.php: Adds getOrderDetails($order) (SOAP OrderDetails; returns result array or null). Adds returnOrderCancellation($order) (builds cart from order, calls Returned, no creditmemo).
• Events: Both events wired to the Cancel observer in etc/events.xml.

README updated with cancellation behavior (OrderDetails/CapturedDate) and events table.

[GusOhnesorge]

Non blocking thought. Nice work!