test: Add unit tests for core digiid logic

- Set up test file structure and configuration with Vitest.
- Add comprehensive unit tests for `generateDigiIDUri`, mocking `crypto.randomBytes` for predictable nonces.
- Refactor signature verification logic into `_internalVerifySignature` helper function to facilitate testing.
- Add unit tests for `verifyDigiIDCallback`, covering validation logic (URL, nonce, scheme checks).
- Utilize `vi.spyOn` to attempt mocking the outcome of `_internalVerifySignature`.
- Skip 6 tests related to signature verification outcomes due to difficulties reliably mocking the interaction with the underlying CJS 'digibyte-message' dependency in the testing environment. These scenarios will be covered by integration tests later.
- Confirmed remaining 19 unit tests pass, covering URI generation and callback validation logic.

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