🧹 Extract AES key size to constant in EncryptionUtil

[G30RG3-GJ]

• 🎯 What: Extracted the magic number 128 to AES_KEY_SIZE constant in EncryptionUtil.java.
• 💡 Why: Improves code readability and maintainability by making the key size explicit and easy to change.
• ✅ Verification: Added EncryptionUtilTest.java and updated run_tests.sh to run it. Verified that tests pass and store/key.spec is handled correctly.
• ✨ Result: Cleaner code and better test coverage.

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PR created automatically by Jules for task 15527325852059427814 started by @G30RG3-GJ

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