zeta: Fix update required notification not showing (#25588)

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

crates/zed/src/zed/inline_completion_registry.rs

@@ -9,6 +9,7 @@ use settings::SettingsStore;
 use std::{cell::RefCell, rc::Rc, sync::Arc};
 use supermaven::{Supermaven, SupermavenCompletionProvider};
 use ui::Window;
+use workspace::Workspace;
 use zeta::{ProviderDataCollection, ZetaInlineCompletionProvider};
 
 pub fn init(client: Arc<Client>, user_store: Entity<UserStore>, cx: &mut App) {
@@ -266,13 +267,13 @@ fn assign_edit_prediction_provider(
                     }
                 }
 
-                let zeta = zeta::Zeta::register(
-                    editor.workspace().map(|w| w.downgrade()),
-                    worktree,
-                    client.clone(),
-                    user_store,
-                    cx,
-                );
+                let workspace = window
+                    .root::<Workspace>()
+                    .flatten()
+                    .map(|workspace| workspace.downgrade());
+
+                let zeta =
+                    zeta::Zeta::register(workspace, worktree, client.clone(), user_store, cx);
 
                 if let Some(buffer) = &singleton_buffer {
                     if buffer.read(cx).file().is_some() {