fix: clean up temp directory in JupyterCodeExecutor.stop()

[r266-tech]

Summary

Fixes #7217

JupyterCodeExecutor creates a temp directory via tempfile.mkdtemp() when no output_dir is provided, but never cleans it up in stop(). This is inconsistent with every other executor in the codebase (LocalCommandLineCodeExecutor, DockerCommandLineCodeExecutor, AzureContainerCodeExecutor) which all properly clean up their temp directories.

Over time, leaked directories and their contents (generated images, HTML files) accumulate on disk.

Changes

• Use tempfile.TemporaryDirectory() instead of tempfile.mkdtemp() when output_dir is None. This provides automatic cleanup via context manager semantics.
• Add cleanup in stop() method: if _temp_dir exists, call cleanup() and set to None.
• Add two tests:
  • test_temp_dir_cleanup_on_stop — verifies temp dir is removed after stop() when no output_dir is provided
  • test_explicit_output_dir_not_cleaned_up — verifies explicitly provided output_dir is preserved

Pattern consistency

This follows the exact pattern used by LocalCommandLineCodeExecutor:

# LocalCommandLineCodeExecutor (working correctly)
self._temp_dir = tempfile.TemporaryDirectory()  # line 255
# ...
self._temp_dir.cleanup()  # line 501

Testing

• Added tests verify both the cleanup case and the non-cleanup case
• All existing tests pass (they all use explicit output_dir, so behavior is unchanged)

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