Function validate_verification_method_multikey 

pub fn validate_verification_method_multikey(
    algorithm: Algorithm,
    binding_type: &str,
    public_key_multibase: &str,
) -> Result<(), AlgorithmError>

Structural validation of a typed public-key binding.

This performs NO cryptography. It checks:

• multikey encoding
• codec ↔ algorithm match
• key length