New testcase for sending large message for cassini

[skinny-b]

A previous PR I mentioned the sending message "was getting limited to 64kb. 'write_all' worked, and I was able to load the db with data from the observer."

We should revisit this to write a test surrounding this case within src/agents/cassini/client/src/lib.rs and possible look into refactoring this code:

                //write message
                let unwrapped_writer = state.writer.clone().unwrap();
                let mut writer = unwrapped_writer.lock().await;
                if let Err(e) = writer.write_all(&buffer).await {
                    error!("Failed to flush stream {e}");
                    return Err(Box::new(e));
                }

Before this was write which can return how much of the message was written that if we want we can refactor to take the size into consideration when sending, to keep sending if it only writes partial amount.