Move CodexAuth and AuthManager to the core crate (#3074)

Fix a long standing layering issue.

codex-rs/core/src/client.rs

@@ -3,9 +3,9 @@ use std::path::Path;
 use std::sync::OnceLock;
 use std::time::Duration;
 
+use crate::AuthManager;
 use bytes::Bytes;
-use codex_login::AuthManager;
-use codex_login::AuthMode;
+use codex_protocol::mcp_protocol::AuthMode;
 use eventsource_stream::Eventsource;
 use futures::prelude::*;
 use regex_lite::Regex;