agent: Support inserting selections as context via @selection (#29045)

WIP

Release Notes:

- N/A

crates/editor/src/editor.rs

@@ -3108,6 +3108,13 @@ impl Editor {
         cx.notify();
     }
 
+    pub fn has_non_empty_selection(&self, cx: &mut App) -> bool {
+        self.selections
+            .all_adjusted(cx)
+            .iter()
+            .any(|selection| !selection.is_empty())
+    }
+
     pub fn has_pending_nonempty_selection(&self) -> bool {
         let pending_nonempty_selection = match self.selections.pending_anchor() {
             Some(Selection { start, end, .. }) => start != end,