Updating fastly.toml loses local_server.pushpin section

**Version**

```
% npx fastly version
Fastly CLI version v11.5.0 (617c3243)
Built with go version go1.24.6 darwin/arm64 (2025-09-08)
Viceroy version: viceroy 0.14.2
```

**What happened**

Invoking something that causes the Fastly CLI to update fastly.toml will wipe out the `local_server.pushpin` section.
