assistant2: Add create buffer tool (#11219)

This PR adds a new tool to the `assistant2` crate that allows the
assistant to create a new buffer with some content.

Release Notes:

- N/A

---------

Co-authored-by: Nathan <nathan@zed.dev>

crates/gpui/src/app/test_context.rs

@@ -218,7 +218,7 @@ impl TestAppContext {
     /// Adds a new window, and returns its root view and a `VisualTestContext` which can be used
     /// as a `WindowContext` for the rest of the test. Typically you would shadow this context with
     /// the returned one. `let (view, cx) = cx.add_window_view(...);`
-    pub fn add_window_view<F, V>(&mut self, build_window: F) -> (View<V>, &mut VisualTestContext)
+    pub fn add_window_view<F, V>(&mut self, build_root_view: F) -> (View<V>, &mut VisualTestContext)
     where
         F: FnOnce(&mut ViewContext<V>) -> V,
         V: 'static + Render,
@@ -230,7 +230,7 @@ impl TestAppContext {
                 bounds: Some(bounds),
                 ..Default::default()
             },
-            |cx| cx.new_view(build_window),
+            |cx| cx.new_view(build_root_view),
         );
         drop(cx);
         let view = window.root_view(self).unwrap();

crates/gpui/src/view.rs

@@ -1,3 +1,4 @@
+use crate::Empty;
 use crate::{
     seal::Sealed, AnyElement, AnyModel, AnyWeakModel, AppContext, Bounds, ContentMask, Element,
     ElementId, Entity, EntityId, Flatten, FocusHandle, FocusableView, GlobalElementId, IntoElement,
@@ -457,3 +458,12 @@ mod any_view {
         view.update(cx, |view, cx| view.render(cx).into_any_element())
     }
 }
+
+/// A view that renders nothing
+pub struct EmptyView;
+
+impl Render for EmptyView {
+    fn render(&mut self, _cx: &mut ViewContext<Self>) -> impl IntoElement {
+        Empty
+    }
+}