add processing_instruction to OrderRequest

[Bala1324]

Title: feat(orders): Add processing_instruction to OrderRequest

Summary

• Added ProcessingInstruction enum with two values:
  • ORDER_COMPLETE_ON_PAYMENT_APPROVAL — PayPal auto-completes the order (authorize/capture) upon
    payer approval
  • NO_INSTRUCTION — Caller manually authorizes or captures after payer approval
• Added optional processingInstruction field to the OrderRequest interface and schema, mapping to
  the processing_instruction JSON key
• Exported ProcessingInstruction from the package index
• Added documentation for the new enum and updated order-request.md

Why

The processing_instruction field is required for payment flows like MBWay, Wero where the order should
be auto-completed on payment approval (ORDER_COMPLETE_ON_PAYMENT_APPROVAL). It was missing from
the OrderRequest interface, making it impossible to type-safely pass this field when creating
orders.