Forward cleanup_kc through execute

[sapunyangkut]

Summary

• add an explicit cleanup_kc option to the top-level execute convenience function
• forward that option to NotebookClient.execute instead of the constructor
• preserve the existing cleanup default when the option is omitted or None
• add regression coverage for an externally supplied kernel manager

Root cause

The convenience wrapper accepted cleanup_kc through **kwargs, so it passed the option to NotebookClient(...). cleanup_kc belongs to execution, not construction, and Traitlets rejected it before execution began.

Impact

Callers that provide a kernel manager can now explicitly request or suppress cleanup through the convenience API. Existing calls keep their current ownership-based default.

AI disclosure

🤖🍆 This change and pull request description were prepared with OpenAI Codex-assisted automation. Codex modified nbclient/client.py and tests/test_client.py for this contribution.

Fixes #348.