fix: accept in-band missing fulfillment errors

[FanouZeng-TT]

Description

test_complete_without_fulfillment in validation_test.py required the
reference server's exact transport posture and message:

self.assert_4xx_error(
  response,
  expected_status=400,
  substring="Fulfillment address and option must be selected",
)

The checkout error model also permits business failures in-band on the existing
resource: a 2xx response with a typed messages[] error while the checkout
remains non-completed. The same test file already handles both postures through
assert_business_error(...) for out-of-stock, unavailable-item, not-found, and
payment failures. The official playground uses code missing for a missing
fulfillment destination.

Fix: use the existing dual-posture helper with missing for the in-band
case and a case-insensitive fulfillment substring for a 4xx rejection.

Category (Required)

• Core Protocol: Changes to the core UCP protocol. (Requires Technical Council approval)
• Governance/Contributing: Changes to governance or contributing processes. (Requires Governance Council approval)
• Capability: New or updated UCP capabilities. (Requires Maintainer approval)
• Documentation: Documentation-only changes. (Requires Maintainer approval)
• Infrastructure: Build, CI, and repository infrastructure. (Requires DevOps Maintainer approval)
• Maintenance: Dependency and 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): Organization-wide community files. (Requires DevOps Maintainer approval)

Related Issues

N/A

Checklist

• I have followed the Contributing Guide and the project's style guidelines.
• 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 or that my feature works.
• 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.

Screenshots / Logs (if applicable)

• validation_test.py: 6/6 passed against the reference Python server
• Full conformance CI loop: all 17 *_test.py files passed
• pre-commit run --all-files: passed