fix: fixture dynamic fulfillment option ids

[FanouZeng-TT]

Description

test_dynamic_fulfillment expected the option IDs exp-ship-us and exp-ship-intl. Those IDs belong to the reference sample, not the fulfillment contract, so a merchant that generated destination-specific options under different IDs failed conformance.

Merged conformance #79 removed the same class of sample literals from the neighboring fulfillment tests and explicitly left this test for a follow-up because it needed a second-destination fixture.

Fix: move both destination/expected-option pairs into the optional dynamic_fulfillment fixture. The test now skips when the merchant declares no dynamic-fulfillment behavior, and otherwise checks the exact domestic and international cases supplied by that merchant. The flower-shop fixture preserves full reference-server coverage.

Category (Required)

• Core Protocol: Changes to core protocol specifications. (Requires Technical Council approval)
• Governance/Contributing: Changes to governance or contributing processes. (Requires Governance Council approval)
• Capability: New or updated capabilities. (Requires Maintainer approval)
• Documentation: Documentation-only changes. (Requires Maintainer approval)
• Infrastructure: Build, CI, or deployment changes. (Requires DevOps Maintainer approval)
• Maintenance: Dependencies 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

Follow-up to #79.

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 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).

Screenshots / Logs (if applicable)

N/A — the 10-test fulfillment module passes against the reference Python server; a custom fixture with merchant-specific IDs and the missing-fixture skip path were verified separately. Ruff, all pre-commit hooks, JSON validation, and git diff --check pass. The broader suite reaches the existing webhook response-model failures tracked by #88, outside these four files.