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

Fix a long standing layering issue.

codex-rs/core/src/model_provider_info.rs

@@ -5,8 +5,8 @@
 //!   2. User-defined entries inside `~/.codex/config.toml` under the `model_providers`
 //!      key. These override or extend the defaults at runtime.
 
-use codex_login::AuthMode;
-use codex_login::CodexAuth;
+use crate::CodexAuth;
+use codex_protocol::mcp_protocol::AuthMode;
 use serde::Deserialize;
 use serde::Serialize;
 use std::collections::HashMap;