Refactor + separately test the instance deletion logic in task scheduling

[isoos]

• Refactored the instance deletion cycle similarly to the package scan logic in Refactor task loop: scanning for package updated timestamps and updating tracking state. #9082.
• Added tests for the cycle's typical use cases.

[isoos]

PTAL: I've updated the code as we've discussed - removed the events, fixed the logic (old test pass), and using a full list of instances.