test(cli): cover completed MPP and x402 payments

[nkavian]

Summary

• Exercise completed MPP and x402 payments through the built CLI and installed SDKs using local seller and platform HTTP fixtures.
• Check transaction creation, pending-to-ready polling, payment headers, POST body preservation, final JSON output, and separation of seller headers from platform credentials.
• Run within the existing pnpm test suite, included in pnpm verify and pull request checks.

No live payments, production behavior changes, dependency updates, or release version bumps.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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