fix: unwrap payment payload in order_test update requests

[XiaolongZhang-TT]

Description

integration_test_utils.get_valid_payment_payload() returns the full request body {"payment": {...}, "risk_signals": {}}, not just the payment object. Two update_checkout payloads in order_test.py assigned this whole dict to the "payment" key, producing:

• a double-nested payment.payment.instruments (the actual instruments buried one level too deep)
• a stray risk_signals field inside the payment object

As a result the server would see an empty/missing instruments list on these updates. The tests currently pass only because the sample server tolerates a missing instruments list on a checkout update that doesn't change payment — a stricter server that validates the payment object on update would fail. Every other call site in the suite uses the helper correctly (as the whole body), so this is the inconsistency.

Fix: extract ["payment"] so the payload matches the contract used everywhere else.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected, including removal of schema files or fields)
• Documentation update

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Is this a Breaking Change or Removal?

N/A — test-only fix, no schema/field removal.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (N/A — test-only fix)
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

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[damaz91]

Reviewed the changes. The fix correctly prevents the double-nested payment payload on checkout update in the two order tests, matching the API schema contract. Confirmed conformance tests pass locally.