Function generate_api_key 

pub fn generate_api_key() -> Result<(String, String), McpxError>

Generate a new API key: 256-bit random token + Argon2id hash.

Returns (plaintext_token, argon2id_hash_phc_string). The plaintext is shown once to the user and never stored.

Errors

Returns an error if salt encoding or Argon2id hashing fails (should not happen with valid inputs, but we avoid panicking).