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/display_map.rs

@@ -391,7 +391,7 @@ impl DisplayMap {
         &mut self,
         crease_ids: impl IntoIterator<Item = CreaseId>,
         cx: &mut Context<Self>,
-    ) {
+    ) -> Vec<(CreaseId, Range<Anchor>)> {
         let snapshot = self.buffer.read(cx).snapshot(cx);
         self.crease_map.remove(crease_ids, &snapshot)
     }