test: make merchant assertions configuration-agnostic

[FanouZeng-TT]

Description

test_idempotency_create uses "EUR" as its conflict currency:

conflict_payload.currency = "EUR"

For a merchant whose fixture currency is already EUR, that produces the same
request body under the same idempotency key. A conforming server replays the
original response, while the test expects 409 for a conflict.

test_fulfillment_flow also derives its expected checkout total from only the
item price and selected shipping option. This disagrees with the suite's own
checkout verification invariant, which sums every non-total entry; a merchant
that adds tax or fees therefore fails an otherwise valid checkout.

Fix: choose a conflict currency distinct from the fixture currency, and
calculate the fulfillment expectation from the complete non-total breakdown.
This keeps the USD reference fixture unchanged while allowing other supported
merchant configurations.

Category (Required)

• Core Protocol: Changes to the base communication layer, global context, or breaking refactors. (Requires Technical Council approval)
• Governance/Contributing: Updates to GOVERNANCE.md, CONTRIBUTING.md, or CODEOWNERS. (Requires Governance Council approval)
• Capability: New schemas (Discovery, Cart, etc.) or extensions. (Requires Maintainer approval)
• Documentation: Updates to README, or documentations regarding schema or capabilities. (Requires Maintainer approval)
• Infrastructure: CI/CD, Linters, or build scripts. (Requires DevOps Maintainer approval)
• Maintenance: Version bumps, lockfile updates, or minor bug fixes. (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 (resolver, linter, validator). (Requires Maintainer approval)
• Community Health (.github): Updates to templates, workflows, or org-level configs. (Requires DevOps Maintainer approval)

Related Issues

Fixes #103

Checklist

• I have followed the Contributing Guide (including Conventional Commits title requirements and ! for breaking changes).
• 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)

N/A — test-only assertion correction.

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