[client-v2] FixedString padding

### Description

Exact number of bytes should be sent when writing a `FixedString`. When string has less number then client add padding `0`s . This behavior is not documented well.

Start in the client-v2 FixedString writing path and trace how the declared string length is converted into bytes. Compare the output with the issue's expected exact length and zero padding, then add coverage or documentation that makes the resulting behavior verifiable.

索引モデルが issue の本文から書いたものです。