assistant2: Fix inline context picker and handle dismiss (#23081)

The new `ContextMenu`-based `ContextPicker` requires initialization when
opened, but we were only doing this for the `ContextStrip` picker, not
the inline one.

Additionally, because we have a wrapper element around ContextMenu, we
need to propagate the `DismissEvent` so that it properly closes when
Escape is pressed.

Release Notes:

- N/A

crates/assistant2/src/context_picker/thread_context_picker.rs

@@ -176,8 +176,7 @@ impl PickerDelegate for ThreadContextPickerDelegate {
 
     fn dismissed(&mut self, cx: &mut ViewContext<Picker<Self>>) {
         self.context_picker
-            .update(cx, |this, cx| {
-                this.reset_mode(cx);
+            .update(cx, |_, cx| {
                 cx.emit(DismissEvent);
             })
             .ok();