assistant2: Make context persistent in the thread (#22789)

This PR makes it so the context is persistent in the thread, rather than
having to reattach it for each message.

This PR intentionally does not make an attempt to refresh the attached
context if it changes. That will come in a follow-up.

Release Notes:

- N/A

crates/assistant2/src/message_editor.rs

@@ -142,7 +142,9 @@ impl MessageEditor {
             editor.clear(cx);
             text
         });
-        let context = self.context_store.update(cx, |this, _cx| this.drain());
+        let context = self
+            .context_store
+            .update(cx, |this, _cx| this.context().clone());
 
         self.thread.update(cx, |thread, cx| {
             thread.insert_user_message(user_message, context, cx);