[client-v2, jdbc-v2] POJO Serialization Issues

### Description

What need to be done:
- Optimize POJO serialization code
- Write more examples and documentation
- Fix know issue
- Increase coverage.

Start by locating the POJO serialization paths in client-v2 and jdbc-v2, then review the existing examples, documentation, and coverage around them. The issue does not name specific files or a failing case, so first identify the known issue and agree on the intended optimization. Done means the serialization behavior is corrected, examples and documentation are expanded, and relevant coverage is increased.

索引モデルが issue の本文から書いたものです。