Keep credentials out of app-server logs (#39993)

## Why

App-server logs can be persisted or included in submitted diagnostics, so credentials used by model providers, authentication refreshes, and attestation requests must not appear in diagnostic output.

## What changed

- Add `RedactedString`, which preserves serialization and string access while replacing debug output with `<redacted>`.
- Use it for model-provider bearer tokens, header and query values, authentication command arguments, and attestation tokens.
- Avoid logging JSON-RPC error payloads and parser or authentication errors that may echo credentials; retain safe context such as error codes and categories.

## Testing

- Add an app-server regression test that exercises provider credentials, refreshed authentication tokens, and attestation tokens, then verifies none appear in persisted SQLite or submitted diagnostic logs.

GitOrigin-RevId: 8c50408adf94d93847658b1320682cf3b637d2cc

codex-rs/app-server/src/message_processor.rs

@@ -829,7 +829,6 @@ impl MessageProcessor {
 
     /// Handle an error object received from the peer.
     pub(crate) async fn process_error(&self, err: JSONRPCError) {
-        tracing::error!("<- error: {:?}", err);
         self.outgoing.notify_client_error(err.id, err.error).await;
     }