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.rs

@@ -66,6 +66,7 @@ pub fn init(
     cx: &mut App,
 ) {
     AssistantSettings::register(cx);
+    thread_store::init(cx);
     assistant_panel::init(cx);
 
     inline_assistant::init(