Use byte size when padding encrypted sessions

[OskarEichler]

Summary

• calculate encrypted-session padding from serialized bytes rather than characters
• preserve the documented byte-multiple contract for multibyte JSON payloads
• leave Marshal/binary payload behavior unchanged

With serialize_json: true, a multibyte value currently produces ciphertext whose payload is not a multiple of pad_size (for example, 17 bytes with pad_size: 16).

Verification

• focused V1/V2 model fails on release/current and passes this candidate with accented and emoji JSON values
• both versions retain successful encrypt/decrypt round trips
• full suite: 192 runs, 307 assertions, 0 failures/errors
• syntax and diff checks pass

[ioquatix]

LGTM.