fix: make fixes to ed25519 sign method + improve tests

[elribonazo]

Description:

After some investigation, we are returning a hex encoded buffer with the signature in JS which causes compatibility issues across SDK and we are also improving the test coverage for ed25519 by using the same values on all platforms we test

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• My PR follows the contribution guidelines of this project
• My PR is free of third-party dependencies that don't comply with the Allowlist
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
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