context_menu: Fix certain menu items not reacting to click events (#49557)

Use `Workspace::for_window` instead of `window.root().flatten()`.



https://github.com/user-attachments/assets/3ea6dae3-166d-49e4-9fff-8256e13cc2bf



https://github.com/user-attachments/assets/eb8d9394-5f71-4df6-b0db-c1c8078f1b9a



Before you mark this PR as ready for review, make sure that you have:
- [x] Added a solid test coverage and/or screenshots from doing manual
testing
- [x] Done a self-review taking into account security and performance
aspects
- [x] Aligned any UI changes with the [UI
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)

Release Notes:

- Fixed certain context menu items not reacting to click events

crates/agent_ui/src/agent_configuration.rs

@@ -983,7 +983,7 @@ impl AgentConfiguration {
                         })
                         .entry("Add Custom Agent", None, {
                             move |window, cx| {
-                                if let Some(workspace) = window.root().flatten() {
+                                if let Some(workspace) = Workspace::for_window(window, cx) {
                                     let workspace = workspace.downgrade();
                                     window
                                         .spawn(cx, async |cx| {

crates/edit_prediction_ui/src/edit_prediction_button.rs

@@ -993,7 +993,7 @@ impl EditPredictionButton {
                         "Edit Prediction Menu Action",
                         action = "configure_excluded_files",
                     );
-                    if let Some(workspace) = window.root().flatten() {
+                    if let Some(workspace) = Workspace::for_window(window, cx) {
                         let workspace = workspace.downgrade();
                         window
                             .spawn(cx, async |cx| {