Add CREDENTIAL_ON_FILE automatic payment example

[luismeli10 (luismeli10)]

Summary

• Adds examples/order/create_order_credential_on_file.py with three complete CREDENTIAL_ON_FILE scenarios using the MercadoPago Orders API
• CIT (Customer-Initiated Transaction / enrolment): type: CREDENTIAL_ON_FILE, sub_type: recurring, storage: store, transaction_initiator: customer, first_transaction: True
• MIT (Merchant-Initiated Transaction / scheduled recurring charge): storage: stored, transaction_initiator: merchant, first_transaction: False, with reference.id pointing to the CIT payment
• UCOF-CIT (Unscheduled Card-On-File, Customer-Initiated): sub_type: unscheduled, storage: stored, transaction_initiator: customer, first_transaction: False

Motivation

The SUBSCRIPTIONS payment type is being replaced by CREDENTIAL_ON_FILE across the MercadoPago platform. This example demonstrates the updated transaction_data block structure that integrators must adopt.

Test plan

• Review the example code for correctness against the API reference
• Confirm the three scenarios cover CIT enrolment, MIT recurring, and UCOF-CIT unscheduled flows
• Validate that placeholder values (YOUR_ACCESS_TOKEN, CUSTOMER_ID, CARD_TOKEN, etc.) are consistent with the existing example style

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