fix: clean up temporary output directory in JupyterCodeExecutor.stop()

[veeceey]

Summary

Fixes #7217

JupyterCodeExecutor creates a temporary directory via tempfile.mkdtemp() when no output_dir is provided, but never cleans it up in stop(). This causes leaked directories and their contents (generated images, HTML files) to accumulate on disk over time.

This is inconsistent with every other executor in the codebase (LocalCommandLineCodeExecutor, DockerCommandLineCodeExecutor, AzureContainerCodeExecutor) which all properly clean up their temporary directories.

Changes

• Replace tempfile.mkdtemp() with tempfile.TemporaryDirectory() in __init__, matching the pattern used by all other executors
• Add cleanup() call in stop() to properly clean up the temporary directory
• User-provided output_dir directories are not affected (only auto-created temp dirs are cleaned up)
• Remove the unused _temp_dir_path field

Tests

Added two new tests:

• test_temp_dir_cleanup_on_stop -- verifies the temporary directory is removed after stop()
• test_user_provided_output_dir_not_cleaned_up -- verifies user-provided directories are preserved after stop()