fix: use checkout line item ids in order fulfillment tests

[FanouZeng-TT]

Description

order_test.py copied the real checkout line items into the update payload:

"line_items": [
  {
    "item": {"id": li.item.id, "title": li.item.title},
    "quantity": li.quantity,
    "id": li.id,
  }
  for li in checkout_obj.line_items
]

The fulfillment method in the same payload still used a fixture-specific fallback:

"line_item_ids": ["item_123"]

That value does not come from the checkout response, so a merchant implementation with different line item IDs can receive a fulfillment selection that references no line item in the current checkout.

Fix: use checkout_obj.line_items[0].id for the fulfillment method and group fallback IDs, matching the line items already sent in the same request.

Category (Required)

• Core Protocol: Changes to the core UCP specification. (Requires Technical Council approval)
• Governance/Contributing: Changes to governance or contributing processes. (Requires Governance Council approval)
• Capability: Changes to a UCP capability. (Requires Maintainer approval)
• Documentation: Documentation-only changes. (Requires Maintainer approval)
• Infrastructure: CI/CD or repository infrastructure changes. (Requires DevOps Maintainer approval)
• Maintenance: Dependency or repository maintenance. (Requires DevOps Maintainer approval)
• SDK: Language-specific SDK updates and releases. (Requires DevOps Maintainer approval)
• Samples / Conformance: Maintaining samples and the conformance suite. (Requires Maintainer approval)
• UCP Schema: Changes to the ucp-schema tool. (Requires Maintainer approval)
• Community Health (.github): Changes to organization community health files. (Requires DevOps Maintainer approval)

Related Issues

N/A

Checklist

• I have followed the Contributing Guide and Code of Conduct.
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• I have added tests that prove my fix is effective.
• New and existing unit tests pass locally with my changes.
• (For Core/Capability) I have included/updated the relevant JSON schemas.
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk (not applicable).

Screenshots / Logs (if applicable)

• uv run python order_test.py --server_url=http://localhost:8182 --simulation_secret=super-secret-sim-key --test_data_dir=test_data/flower_shop: 4 passed
• pre-commit run --all-files: passed
• git diff --check: passed