🔒 Fix deterministic AES encryption (weak zero nonce) in crypto stdlib

[Tcode-Motion]

🎯 What: Fixed a vulnerability in stdlib/src/crypto.rs where the aes_encrypt function used a hardcoded, deterministic 12-byte zero nonce. This has been replaced with a secure, random 12-byte nonce generated using rand::thread_rng().

⚠️ Risk: Using a zero nonce made the AES-GCM encryption deterministic. Encrypting multiple messages with the same key using a reused nonce completely breaks AES-GCM security, exposing the system to severe attacks (such as the recovery of the authentication key and decryption of ciphertext).

🛡️ Solution:

• Modified aes_encrypt to securely generate a random 12-byte nonce using the rand crate.
• Updated aes_encrypt to prepend the 12-byte nonce to the encrypted payload before hex encoding, matching industry standard practice for AES-GCM.
• Modified aes_decrypt to extract the 12-byte nonce from the beginning of the decoded ciphertext payload, ensuring decryption works dynamically.
• Added comprehensive unit tests in stdlib_tests.rs to verify that two identical plaintexts encrypted with the same key yield different ciphertexts, validating the non-deterministic nature of the fix.

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PR created automatically by Jules for task 2807267893354396822 started by @Tcode-Motion

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