assistant2: Push logic for adding file context down into the ContextStore (#22846)

This PR takes the logic for adding file context out of the
`FileContextPicker` and pushes it down into the `ContextStore`.

Release Notes:

- N/A

crates/assistant2/src/message_editor.rs

@@ -47,7 +47,7 @@ impl MessageEditor {
         thread: Model<Thread>,
         cx: &mut ViewContext<Self>,
     ) -> Self {
-        let context_store = cx.new_model(|_cx| ContextStore::new());
+        let context_store = cx.new_model(|_cx| ContextStore::new(workspace.clone()));
         let context_picker_menu_handle = PopoverMenuHandle::default();
         let inline_context_picker_menu_handle = PopoverMenuHandle::default();
         let model_selector_menu_handle = PopoverMenuHandle::default();