The API schema export is currently broken:
```
> cargo run -p codex-app-server-protocol --bin export -- --out DIR
Error: this type cannot be exported
```
This PR fixes the error message so we get more info:
```
> cargo run -p codex-app-server-protocol --bin export -- --out DIR
Error: failed to export client responses: dependency core::option::Option<codex_protocol::account::Account> cannot be exported
```
And fixes the root cause which is the `account/read` response.