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/agent/src/inline_assistant.rs

@@ -1199,6 +1199,7 @@ impl InlineAssistant {
     ) -> Vec<InlineAssistId> {
         let assist_group = self.assist_groups.get_mut(&assist_group_id).unwrap();
         assist_group.linked = false;
+
         for assist_id in &assist_group.assist_ids {
             let assist = self.assists.get_mut(assist_id).unwrap();
             if let Some(editor_decorations) = assist.decorations.as_ref() {