Fix cleanup of JupyterCodeExecutor temp output dir

[zerone0x]

Summary

• ensure the Jupyter code executor recreates and removes its managed temporary output directory when stopping
• cover the default path cleanup with a regression test

Testing

• .venv/bin/python -m pytest python/packages/autogen-ext/tests/code_executors/test_jupyter_code_executor.py -k output_dir

Fixes #7217

[zerone0x]

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