Function subscribe_error_frame 

pub fn subscribe_error_frame(
    stream_id: u32,
    error: &ProtocolError,
) -> Result<Frame, ProtocolError>

Construct a SubscribeError frame for a failed subscription negotiation.

This keeps schema incompatibility visible on the subscription stream as an explicit protocol error frame with the stable numeric reason code carried by ProtocolError::reason_code.

Errors

Returns ProtocolError::InvalidStream when stream_id is not an application stream.