Function delete_message 

pub async fn delete_message<T: Transport + ?Sized>(
    transport: &T,
    message_id: &str,
) -> Result<(), String>

Delete a message the local user previously sent, by its INNER message id (what the UI holds). Loads the retained ephemeral key + the outer event id it points at, then NIP-09-deletes that outer event. Errors if no key is retained (not ours, or already deleted).