Provider-backed agent delete is routed through persona deletion and fails

**Describe the bug**

Deleting a provider-backed agent from the Agents card opens the persona deletion flow. The dialog says it will also delete the linked agent instance, but confirmation fails with:

```text
persona has provider-deployed agent instances (); delete those agent instances first
```

**Steps to reproduce**

1. Create an agent using an external provider.
2. Open the agent card’s `⋮` menu.
3. Select **Delete** and confirm.

**Expected behavior**

The action deletes the provider-backed agent instance, or directs the user to the instance deletion action.

**Actual behavior**

It enters the persona cascade-delete path, promises to delete the instance, then refuses because the instance is provider-deployed.

Possibly related: #2798.

**Version and platform**

- Buzz: 0.5.2
- macOS: 26.5.2