Support staging OAuth client ID overrides (#28257)

## Summary

- allow app-server ChatGPT login to use a configured OAuth client ID
- reuse the same client ID for refresh and revoke requests
- cover staging login, refresh, and revoke request payloads

## Tests

- `just test -p codex-login`
- `just test -p codex-app-server
login_account_chatgpt_uses_debug_oauth_overrides`
- `just test -p codex-login
logout_with_revoke_revokes_refresh_token_then_removes_auth`
- `just fix -p codex-login`
- `just fix -p codex-app-server`
- `just fmt`

codex-rs/login/src/lib.rs

@@ -23,6 +23,7 @@ pub use auth::AuthKeyringBackendKind;
 pub use auth::AuthManager;
 pub use auth::AuthManagerConfig;
 pub use auth::CLIENT_ID;
+pub use auth::CLIENT_ID_OVERRIDE_ENV_VAR;
 pub use auth::CODEX_ACCESS_TOKEN_ENV_VAR;
 pub use auth::CODEX_API_KEY_ENV_VAR;
 pub use auth::CodexAuth;
@@ -45,6 +46,7 @@ pub use auth::login_with_api_key;
 pub use auth::login_with_bedrock_api_key;
 pub use auth::logout;
 pub use auth::logout_with_revoke;
+pub use auth::oauth_client_id;
 pub use auth::read_codex_access_token_from_env;
 pub use auth::read_openai_api_key_from_env;
 pub use auth::save_auth;