Missing Google-Style Docstrings in SparkClient API

Currently, the SparkClient class in kubeflow/spark/api/spark_client.py has incomplete docstrings for several of its methods. According to the repository guidelines defined in AGENTS.md (under "Documentation Standards"), all public functions should include detailed Google-style docstrings with Args, Returns, and Raises sections as appropriate.

The current implementation lacks complete docstrings for the following methods:

`list_sessions`: Missing the Returns section.
`get_session`: Missing the Args, Returns, and Raises sections.
`delete_session`: Missing the Args and Raises sections.
`get_session_logs`: Missing the Args, Returns, and Raises sections.

Steps to Reproduce:
1. Navigate to kubeflow/spark/api/spark_client.py
2. Check methods `list_sessions`, `get_session`, `delete_session`, and `get_session_logs`.