Re-add MultiWorkspace (#48800)

Release Notes:

- Added agent panel restoration. Now restarting your editor won't cause
your thread to be forgotten.

---------

Co-authored-by: Anthony Eid <56899983+Anthony-Eid@users.noreply.github.com>
Co-authored-by: Eric Holk <eric@zed.dev>
Co-authored-by: Danilo Leal <67129314+danilo-leal@users.noreply.github.com>
Co-authored-by: Anthony Eid <anthony@zed.dev>
Co-authored-by: Mikayla Maki <mikayla.c.maki@gmail.com>
Co-authored-by: Cameron Mcloughlin <cameron.studdstreet@gmail.com>

crates/git_ui/src/text_diff_view.rs

@@ -450,6 +450,7 @@ mod tests {
     use settings::SettingsStore;
     use unindent::unindent;
     use util::{path, test::marked_text_ranges};
+    use workspace::MultiWorkspace;
 
     fn init_test(cx: &mut TestAppContext) {
         cx.update(|cx| {
@@ -675,8 +676,9 @@ mod tests {
 
         let project = Project::test(fs, [project_root.as_ref()], cx).await;
 
-        let (workspace, cx) =
-            cx.add_window_view(|window, cx| Workspace::test_new(project.clone(), window, cx));
+        let (multi_workspace, cx) =
+            cx.add_window_view(|window, cx| MultiWorkspace::test_new(project.clone(), window, cx));
+        let workspace = multi_workspace.read_with(cx, |mw, _| mw.workspace().clone());
 
         let buffer = project
             .update(cx, |project, cx| project.open_local_buffer(file_path, cx))