Function sha256_hash 

pub fn sha256_hash(username: &[u8], password: &[u8], salt: &[u8]) -> String

Hashes authentication information using SHA256_MD5 for GaussDB/OpenGauss.

This implements the SHA256_MD5 authentication method used by GaussDB and OpenGauss. The process is: MD5(password + username) -> SHA256(md5_hex + salt) -> “sha256” + hex