fix: use dynamic expected/actual counts in toBytes error

[Bashmunta]

Replace hardcoded "output must be 8 elements" with a dynamic error that reports the expected number of outputs (nbLimbs) and the actual length. This aligns with conventions across other hints, prevents misleading errors for U32 (4 outputs), and improves diagnostics.

---

Note

Replace hardcoded output count error in toBytes with a dynamic message reporting expected and actual counts.

^{Written by Cursor Bugbot for commit 45d26a8. This will update automatically on new commits. Configure here.}

[ivokub]

Thanks for the contribution. It is a small change and will be merged to master branch through a separate branch to reduce CI usage.