🧪 Test shutdown_image_executors() for proper thread pool cleanup

[marko1olo]

🎯 What: Added unit tests to test the shutdown_image_executors function in Dubsite_tgach/image_processing.py.
📊 Coverage: Tests verify the three main branches: both executor pools exist, one executor pool exists, and both executor pools are None. It validates that .shutdown() is correctly called and that global executor properties are properly cleared out.
✨ Result: Improved test coverage and ensured regression safety for thread pool cleanup execution logic.

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PR created automatically by Jules for task 13707712160485298518 started by @marko1olo

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