Function generate_api_key 

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

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 Argon2id hashing fails (should not happen with valid inputs, but we avoid panicking).