acp: Clean up entry views on rewind (#36197)

We were leaking diffs and terminals on rewind, we'll now clean them up.
This PR also introduces a refactor of how we mantain the entry view
state to use a `Vec` that's kept in sync with the thread entries.

Release Notes:

- N/A

crates/agent2/src/tests/mod.rs

@@ -841,7 +841,7 @@ async fn test_agent_connection(cx: &mut TestAppContext) {
     // Create a thread using new_thread
     let connection_rc = Rc::new(connection.clone());
     let acp_thread = cx
-        .update(|cx| connection_rc.new_thread(project, cwd, &mut cx.to_async()))
+        .update(|cx| connection_rc.new_thread(project, cwd, cx))
         .await
         .expect("new_thread should succeed");