feat(codex): preserve OAuth login state during third-party provider switching

Codex provider switches now only write config.toml for third-party providers,
injecting the API key as experimental_bearer_token. The user's auth.json
(ChatGPT OAuth tokens) is preserved. Official providers with login material
still write auth.json normally. Backfill restores bearer tokens into stored
provider auth.OPENAI_API_KEY to maintain canonical shape.

src-tauri/src/commands/config.rs

@@ -82,7 +82,8 @@ pub async fn get_config_status(
         }
         AppType::Codex => {
             let auth_path = codex_config::get_codex_auth_path();
-            let exists = auth_path.exists();
+            let config_text = codex_config::read_codex_config_text().unwrap_or_default();
+            let exists = auth_path.exists() || !config_text.trim().is_empty();
             let path = codex_config::get_codex_config_dir()
                 .to_string_lossy()
                 .to_string();