assistant2: Fix thread history only working in one Zed window (#25119)

This PR fixes an issue where the thread history would only work in one
Zed window at a time.

The backing LMDB database can only be opened once per Zed instance.
However, the `ThreadStore` has one instance per Zed window.

To fix this, we need to create the `heed` environment once and store it
as a global, and then reference the same environment across all of the
`ThreadStore`s.

Release Notes:

- N/A

crates/assistant2/src/assistant_panel.rs

@@ -323,6 +323,9 @@ impl AssistantPanel {
     }
 
     fn open_history(&mut self, window: &mut Window, cx: &mut Context<Self>) {
+        self.thread_store
+            .update(cx, |thread_store, cx| thread_store.reload(cx))
+            .detach_and_log_err(cx);
         self.active_view = ActiveView::History;
         self.history.focus_handle(cx).focus(window);
         cx.notify();