test: fix unhandled promise rejection

[Uzlopak]

fixes following

stderr | test/sign.test.ts > sign > throws with eventPayload as object
Promise returned by `expect(actual).rejects.toThrow(expected)` was not awaited. Vitest currently auto-awaits hanging assertions at the end of the test, but this will cause the test to fail in Vitest 3. Please remember to await the assertion.
    at /home/aras/workspace/webhooks-methods.js/test/sign.test.ts:52:44

[github-actions]

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[github-actions]

🎉 This PR is included in version 6.0.0 🎉

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