Function check_decode_policy_admission 

pub fn check_decode_policy_admission<D: DecoderConfig + 'static>(
    dec: D,
    bytes: &[u8],
    initial: DecodePolicy,
    request: DecodePolicy,
    accepted: bool,
    expected: &Metadata,
) -> Conformance

Verify checked decode-policy admission, typed rejection, and unchanged output after rejection. Supply explicit expected metadata to test supported metadata denials. All paths decode the same caller-provided fixture.