Dequalify WindowContext and AsyncWindowContext identifiers (#22444)

This makes a WIP automated refactoring a bit easier to write

Release Notes:

- N/A

crates/assistant2/src/context_picker/fetch_context_picker.rs

@@ -176,7 +176,7 @@ impl PickerDelegate for FetchContextPickerDelegate {
 
     fn set_selected_index(&mut self, _ix: usize, _cx: &mut ViewContext<Picker<Self>>) {}
 
-    fn placeholder_text(&self, _cx: &mut ui::WindowContext) -> Arc<str> {
+    fn placeholder_text(&self, _cx: &mut WindowContext) -> Arc<str> {
         "Enter a URL…".into()
     }