This PR fixes an issue introduced in #25530 that broke the notifications
that inform the user that a Zed update is required to continue using
edit prediction.

The issue is that the `Workspace` stored on the `Editor` is set _after_
the point we initialize Zeta, so capturing the `Workspace` at
construction time leads to it being `None`.

@ConradIrwin suggested that we could obtain the `Workspace` from the
`Window`, which does indeed do the trick.

I tested it both with and without this change by mocking the error
response, like so:

```rs
let response: Result<PredictEditsResponse, anyhow::Error> =
    Err(anyhow!(ZedUpdateRequiredError {
        minimum_version: SemanticVersion::new(0, 1, 0),
    }));
```

Release Notes:

- N/A