fix: use consistent RNG source for ECIES nonce generation

[viktorking7]

Description

Problem: ECIES initialization was mixing randomness sources by using B256::random() for nonce generation while using a local RNG for ephemeral keys. This creates inconsistent entropy sources in cryptographic operations.

Solution: Generate nonce using the same local RNG instance that's already created for ephemeral key generation.

Changes:

• Replace B256::random() with rng.fill_bytes() in new_client() and new_server()
• Add RngCore import for fill_bytes() method