Fix lag when interacting with MarkdownElement (#31585)

Previously, we forgot to associate the `Markdown` entity to
`MarkdownElement` during `prepaint`. This caused calls to
`Context<Markdown>::notify` to not invalidate the view cache, which
meant we would have to wait for some other invalidation before seeing
the results of that initial notify.

Release Notes:

- Improved responsiveness of mouse interactions with the agent panel.

Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>

crates/markdown/src/markdown.rs

@@ -1209,6 +1209,7 @@ impl Element for MarkdownElement {
     ) -> Self::PrepaintState {
         let focus_handle = self.markdown.read(cx).focus_handle.clone();
         window.set_focus_handle(&focus_handle, cx);
+        window.set_view_id(self.markdown.entity_id());
 
         let hitbox = window.insert_hitbox(bounds, false);
         rendered_markdown.element.prepaint(window, cx);