Add backend client support for Codex user settings (#34559)

## What changed

- Add `Client::get_user_settings` for the active Codex API or ChatGPT backend route.
- Send authenticated workspace headers and bypass cached responses when fetching settings.
- Expose the effective `commit_attribution_enabled` policy, defaulting it to `false` when older responses omit the field.

## Testing

- Cover both backend route styles, cache-control behavior, response decoding, the compatibility default, and authenticated workspace headers.

GitOrigin-RevId: 11db485e89293f9fa9c56ac7997922deed8b9427

codex-rs/backend-client/src/lib.rs

@@ -8,6 +8,7 @@ pub use types::AccountEntry;
 pub use types::AccountsCheckResponse;
 pub use types::CodeTaskDetailsResponse;
 pub use types::CodeTaskDetailsResponseExt;
+pub use types::CodexUserSettingsResponse;
 pub use types::CodexWorkspaceMessage;
 pub use types::CodexWorkspaceMessageType;
 pub use types::CodexWorkspaceMessagesResponse;