🧪 Add test for cache update in find_or_create_folder

[yj9404]

🎯 What: Added a missing unit test to verify that _FOLDER_CACHE is updated successfully after a new folder is created in the find_or_create_folder function.
📊 Coverage: Specifically covers the happy path inside find_or_create_folder where r.status_code in (200, 201) and the newly returned folder JSON object is expected to be appended to the cached list for parent_id.
✨ Result: Improved test coverage and reliability for folder creation logic, preventing future regressions by explicitly verifying state mutation of the global cache dict.

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PR created automatically by Jules for task 196617528914632719 started by @yj9404

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