feat(tools): support IPython persistent shell for variable sharing

[Lightblues]

Summary

• Add persistent IPython shell instance in PythonExecutorToolkit for local mode
• Enable variable sharing across multiple code executions
• Provide true interactive IPython experience with stateful execution

Changes

• Modified utu/tools/local_env/python.py:
  • Added create_ipython_shell() and cleanup_ipython_shell() helper functions
  • Updated execute_python_code_sync() to support optional shell parameter for reuse
  • Updated execute_python_code_async() to pass shell instance through
• Modified utu/tools/python_executor_toolkit.py:
  • Added _ipython_shell instance variable to maintain persistent shell in local mode
  • Updated execute_python_code() to pass shell instance for variable sharing
  • Added cleanup() method to properly clean up shell resources
• Added comprehensive tests in tests/tools/test_python_executor_toolkit.py

Closes #239