test: improve unit tests coverage for AccountService (Infection)

[diogopaz]

Part of #8053

📝 Summary

This PR adds extensive PHPUnit test coverage to AccountServiceTest to eliminate escaped mutants and improve the mutation testing score with Infection, as part of #8053.

🧪 How to test

Run the focused unit test suite for AccountService:

composer test:unit -- --filter AccountServiceTest

Optionally run Infection on this specific pair to verify mutant coverage:

vendor-bin/infection/vendor/bin/infection --filter=AccountService

⚙️ API / Back‑end changes

• Unit tests added to cover validateCreateToSign, getFileByUuid, createToSign, saveVisibleElement(s), uploadPfx, updatePfxPassword, readPfxData, and config fallbacks.
• Unit and/or integration tests added – required for backend changes

✅ Checklist

• I have read and followed the contribution guide.

🤖 AI (if applicable)

• The content of this PR was partially or fully generated using AI

[welcome]

Thanks for opening your first pull request in this repository! ✌️

[vitormattos]

@diogopaz the problems in the unit tests aren't caused by your PR. I'll fix this in a separate PR.

But have linter issues that you need to solve:

https://github.com/LibreSign/libresign/actions/runs/33325571740/job/100018743935?pr=8142

[vitormattos]

Once you fix the linter issue and push the changes, the unit tests will work fine.