feat(order): add AP dataclasses, docs and example for automatic payments (IXE-679)

[danielalfarourrea]

Summary

New dataclasses for the Automatic Payments flow — optional typed helpers; the existing dict-based API is unchanged.

New files (mercadopago/resources/):

• order_stored_credential.py → OrderStoredCredential — includes prev_transaction_ref, required from the second recurring charge onwards.
• order_automatic_payments.py → OrderAutomaticPayments
• order_subscription_data.py → OrderSubscriptionData + OrderSubscriptionSequence + OrderInvoicePeriod
• order_integration_data.py → OrderIntegrationData + OrderSponsor

Updated:

• order.py → class-level docstring documenting all AP dict keys with types.

New example:

• examples/order/create_order_automatic_payment.py → full AP flow: first payment (first_payment=True) and recurring charge (prev_transaction_ref=prev_tx_id).

How to use

import dataclasses
from mercadopago.resources.order_stored_credential import OrderStoredCredential

# Build typed object
sc = OrderStoredCredential(
    payment_initiator="merchant",
    reason="recurring",
    first_payment=False,
    prev_transaction_ref="TX-001-2026"
)

# Convert to dict and pass to SDK
sdk.order().create({
    "transactions": {"payments": [{
        "stored_credential": dataclasses.asdict(sc)
    }]}
})

Both approaches (plain dict and dataclass) produce identical JSON. The dataclass is optional.

Context

Part of IXE-679: fields required for the Automatic Payments flow in the Orders API were missing from the public SDKs. Companion PRs exist for Java, Go, .NET, Node.js, PHP and Ruby.

Test plan

• python -m py_compile on all new files — OK
• Integration tests (not run per policy — require ACCESS_TOKEN)

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