gpui: Simplify uniform list API by removing entity param (#32480)

This PR also introduces `Context::processor`, a sibling of
`Context::listener` that takes a strong pointer to entity and allows for
a return result.

Release Notes:

- N/A

Co-authored-by: Mikayla <mikayla@zed.dev>

crates/agent/src/thread_history.rs

@@ -594,10 +594,11 @@ impl Render for ThreadHistory {
                     view.pr_5()
                         .child(
                             uniform_list(
-                                cx.entity().clone(),
                                 "thread-history",
                                 self.list_item_count(),
-                                Self::list_items,
+                                cx.processor(|this, range: Range<usize>, window, cx| {
+                                    this.list_items(range, window, cx)
+                                }),
                             )
                             .p_1()
                             .track_scroll(self.scroll_handle.clone())