Update Agent panel to work with CloudUserStore (#35436)

This PR updates the Agent panel to work with the `CloudUserStore`
instead of the `UserStore`, reducing its reliance on being connected to
Collab to function.

Release Notes:

- N/A

---------

Co-authored-by: Richard Feldman <oss@rtfeldman.com>

crates/agent_ui/src/active_thread.rs

@@ -3820,6 +3820,7 @@ mod tests {
     use super::*;
     use agent::{MessageSegment, context::ContextLoadResult, thread_store};
     use assistant_tool::{ToolRegistry, ToolWorkingSet};
+    use client::CloudUserStore;
     use editor::EditorSettings;
     use fs::FakeFs;
     use gpui::{AppContext, TestAppContext, VisualTestContext};
@@ -4116,10 +4117,16 @@ mod tests {
         let (workspace, cx) =
             cx.add_window_view(|window, cx| Workspace::test_new(project.clone(), window, cx));
 
+        let (client, user_store) =
+            project.read_with(cx, |project, _cx| (project.client(), project.user_store()));
+        let cloud_user_store =
+            cx.new(|cx| CloudUserStore::new(client.cloud_client(), user_store, cx));
+
         let thread_store = cx
             .update(|_, cx| {
                 ThreadStore::load(
                     project.clone(),
+                    cloud_user_store,
                     cx.new(|_| ToolWorkingSet::default()),
                     None,
                     Arc::new(PromptBuilder::new(None).unwrap()),