chore: move top-level load_auth() to CodexAuth::from_codex_home() (#1966)

There are two valid ways to create an instance of `CodexAuth`:
`from_api_key()` and `from_codex_home()`. Now both are static methods of
`CodexAuth` and are listed first in the implementation.

---
[//]: # (BEGIN SAPLING FOOTER)
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with [ReviewStack](https://reviewstack.dev/openai/codex/pull/1966).
* #1971
* #1970
* __->__ #1966
* #1965
* #1962

codex-rs/tui/src/lib.rs

@@ -12,7 +12,7 @@ use codex_core::config::load_config_as_toml_with_cli_overrides;
 use codex_core::config_types::SandboxMode;
 use codex_core::protocol::AskForApproval;
 use codex_core::protocol::SandboxPolicy;
-use codex_login::load_auth;
+use codex_login::CodexAuth;
 use codex_ollama::DEFAULT_OSS_MODEL;
 use log_layer::TuiLogLayer;
 use std::fs::OpenOptions;
@@ -304,7 +304,7 @@ fn should_show_login_screen(config: &Config) -> bool {
         // Reading the OpenAI API key is an async operation because it may need
         // to refresh the token. Block on it.
         let codex_home = config.codex_home.clone();
-        match load_auth(&codex_home) {
+        match CodexAuth::from_codex_home(&codex_home) {
             Ok(Some(_)) => false,
             Ok(None) => true,
             Err(err) => {