[TEST] Implement Unit Tests for Encryption/Decryption flow

[aarchiev]

Problem

The project currently lacks automated tests. Refactoring code poses a risk of breaking the encryption logic.

Solution

Create a test module in main.rs or a separate tests/ directory.

Tests Needed

1. Roundtrip Test: Encrypt a dummy text -> Decrypt it -> Assert strict equality.
2. Wrong Password Test: Ensure decryption fails gracefully with an incorrect key.
3. Corrupt Data Test: Modify one byte of the encrypted stream and ensure Poly1305 MAC check fails.