🧪 [Testing Improvement] Add test for ArrayList pop keeping capacity

[void0x14]

🎯 What: Added comprehensive test for ArrayList.pop addressing the testing gap regarding capacity retention and null return on empty.
📊 Coverage: Now tests appending items, popping them, checking the returned items and length, asserting that the underlying capacity does not shrink upon popping, and checking the null return when the list is fully emptied.
✨ Result: Improved test coverage and reliability for arraylist pop edge cases and memory behavior.

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PR created automatically by Jules for task 15071322874735489290 started by @ulac000000

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