Support @-mentions in inline assists and when editing old agent panel messages (#29734)

Closes #ISSUE

Co-authored-by: Bennet <bennet@zed.dev>

Release Notes:

- Added support for context `@mentions` in the inline prompt editor and
when editing past messages in the agent panel.

---------

Co-authored-by: Bennet Bo Fenner <bennet@zed.dev>
Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>

crates/editor/src/editor.rs

@@ -16239,9 +16239,9 @@ impl Editor {
         &mut self,
         ids: impl IntoIterator<Item = CreaseId>,
         cx: &mut Context<Self>,
-    ) {
+    ) -> Vec<(CreaseId, Range<Anchor>)> {
         self.display_map
-            .update(cx, |map, cx| map.remove_creases(ids, cx));
+            .update(cx, |map, cx| map.remove_creases(ids, cx))
     }
 
     pub fn longest_row(&self, cx: &mut App) -> DisplayRow {