fix: use configured customer fixture in webhook address tests - #86

Description

The two webhook address tests hardcoded the flower-shop customer instead of
using the configured fixture:

buyer_info = {"fullName": "John Doe", "email": "john.doe@example.com"}
...
self.assertEqual(expectations[0]["destination"]["address_country"], "US")

--fixture_config is a supported conformance input, and
DynamicFixtureContext.get_known_customer() already provides the customer a
merchant declares. A valid configuration with a different customer would send
the wrong buyer, while a selected non-US destination would be rejected even
when the webhook reproduced it correctly.

Fix: build both buyers from get_known_customer(), skip the tests when the
optional fixture is absent, and compare the webhook destination with the actual
destination selected from the checkout response rather than a hardcoded
country. The explicit new-address assertions (CA / Webhook St) remain
unchanged.

Category (Required)

• Core Protocol: ...
• Governance/Contributing: ...
• Capability: ...
• Documentation: ...
• Infrastructure: ...
• Maintenance: ...
• SDK: ...
• Samples / Conformance: Maintaining samples and the conformance suite. (Requires Maintainer approval)
• UCP Schema: ...

Related Issues

N/A

Checklist

• I have followed the Contributing Guide ...
• 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. (Not applicable: conformance test-only change.)

Screenshots / Logs (if applicable)

N/A — webhook_test.py passed 3/3 against the local Python sample merchant server. ruff check, ruff format --check, full pre-commit, and git diff --check also passed.