chore: make CodexAuth::api_key a private field (#1965)

Force callers to access this information via `get_token()` rather than
messing with it directly.

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* #1971
* #1970
* #1966
* __->__ #1965
* #1962

codex-rs/login/src/lib.rs

@@ -38,8 +38,9 @@ pub enum AuthMode {
 
 #[derive(Debug, Clone)]
 pub struct CodexAuth {
-    pub api_key: Option<String>,
     pub mode: AuthMode,
+
+    api_key: Option<String>,
     auth_dot_json: Arc<Mutex<Option<AuthDotJson>>>,
     auth_file: PathBuf,
 }