This [change](https://github.com/agentclientprotocol/registry/pull/34)
broke the ACP registry in Zed, since we did not have `#[serde(default)]`
on the `_extensions` field.
- [x] Tests or screenshots needed?
- [x] Code Reviewed
- [x] Manual QA
Release Notes:
- Fixed an issue where the ACP registry would fail to load