test(rest/python): adapt checkout fixture to ucp-sdk 0.4.6

[WenshuangQuan-TT]

Description

Problem

The shared checkout payload fixture in rest/python/server/integration_test.py builds a FulfillmentMethodCreateRequest with the response model ShippingDestination.

A fresh sync currently resolves ucp-sdk 0.4.6. Its generated destinations field validates operation-specific request models, so passing ShippingDestination raises a Pydantic ValidationError before any HTTP request is sent. This breaks the Python integration tests that reuse _create_checkout_payload.

Change

Import and construct ShippingDestinationCreateRequest in the checkout-create fixture so the nested model matches FulfillmentMethodCreateRequest.destinations.

This is a test-fixture compatibility fix only. It does not change merchant-server runtime behavior, protocol schemas, or API responses.

Category (Required)

• Core Protocol
• Governance/Contributing
• Capability
• Documentation
• Infrastructure
• Maintenance
• SDK
• Samples / Conformance
• UCP Schema
• Community Health (.github)

Related Issues

None.

Verification

Focused reproduction with ucp-sdk 0.4.6:

baseline ShippingDestination: ValidationError
fixed ShippingDestinationCreateRequest: accepted

Full local checks:

pytest -q
245 passed, 3 warnings, 12 subtests passed

ruff check integration_test.py
ruff format --check integration_test.py
git diff --check

Checklist

• I followed the contributing guide and used a Conventional Commits title.
• Documentation is not applicable because this only changes a test fixture.
• Local linting and formatting checks pass.
• Existing integration tests directly reproduce and verify the fix.
• New and existing tests pass locally.
• Schema changes and SDK model regeneration are not applicable.