[Auth] Introduce New Auth Storage Abstraction for Codex CLI (#5569)

This PR introduces a new `Auth Storage` abstraction layer that takes
care of read, write, and load of auth tokens based on the
AuthCredentialsStoreMode. It is similar to how we handle MCP client
oauth
[here](https://github.com/openai/codex/blob/main/codex-rs/rmcp-client/src/oauth.rs).
Instead of reading and writing directly from disk for auth tokens, Codex
CLI workflows now should instead use this auth storage using the public
helper functions.

This PR is just a refactor of the current code so the behavior stays the
same. We will add support for keyring and hybrid mode in follow-up PRs.

I have read the CLA Document and I hereby sign the CLA

codex-rs/core/tests/suite/client.rs

@@ -525,7 +525,7 @@ async fn prefers_apikey_when_config_prefers_apikey_even_with_chatgpt_tokens() {
     let mut config = load_default_config_for_test(&codex_home);
     config.model_provider = model_provider;
 
-    let auth_manager = match CodexAuth::from_codex_home(codex_home.path()) {
+    let auth_manager = match CodexAuth::from_auth_storage(codex_home.path()) {
         Ok(Some(auth)) => codex_core::AuthManager::from_auth_for_testing(auth),
         Ok(None) => panic!("No CodexAuth found in codex_home"),
         Err(e) => panic!("Failed to load CodexAuth: {e}"),