🧪 testing improvement: idleSchedule test coverage

[MarsLuay]

🎯 What: Improved test coverage for idleSchedule.ts by adding tests for missing edge cases.
📊 Coverage: Added test cases for missing cancelIdleCallback, handling of fallback timeouts greater than 250ms, and missing options fallback default logic.
✨ Result: Test coverage for idleSchedule.ts improved from partial to complete, and functionality is verified to be robust under fallback and missing-API conditions.

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PR created automatically by Jules for task 606558203251274695 started by @MarsLuay

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