fix: prevent possible panic from copy_from_slice in HashableKey::bytes32_raw

[kien-rise]

Motivation

let vkey_bytes = vkey_digest_bn254.as_canonical_biguint().to_bytes_be();
...
result[1..].copy_from_slice(&vkey_bytes);

It's not guaranteed that vkey_bytes has 31 bytes.
If it has 30 bytes, or less, .copy_from_slice will panic.

Solution

Use the correct offset.

PR Checklist

• Added Tests
• Added Documentation
• Breaking changes