thread_view: Move handlers for confirmed completions to the MessageEditor (#36214)

Release Notes:

- N/A

---------

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

crates/editor/src/editor.rs

@@ -2369,6 +2369,34 @@ impl Editor {
             .is_some_and(|menu| menu.context_menu.focus_handle(cx).is_focused(window))
     }
 
+    pub fn is_range_selected(&mut self, range: &Range<Anchor>, cx: &mut Context<Self>) -> bool {
+        if self
+            .selections
+            .pending
+            .as_ref()
+            .is_some_and(|pending_selection| {
+                let snapshot = self.buffer().read(cx).snapshot(cx);
+                pending_selection
+                    .selection
+                    .range()
+                    .includes(&range, &snapshot)
+            })
+        {
+            return true;
+        }
+
+        self.selections
+            .disjoint_in_range::<usize>(range.clone(), cx)
+            .into_iter()
+            .any(|selection| {
+                // This is needed to cover a corner case, if we just check for an existing
+                // selection in the fold range, having a cursor at the start of the fold
+                // marks it as selected. Non-empty selections don't cause this.
+                let length = selection.end - selection.start;
+                length > 0
+            })
+    }
+
     pub fn key_context(&self, window: &Window, cx: &App) -> KeyContext {
         self.key_context_internal(self.has_active_edit_prediction(), window, cx)
     }