Show recently-opened files when autocompleting /file without arguments (#12434)

<img width="1588" alt="image"
src="https://github.com/zed-industries/zed/assets/482957/ea63b046-64d6-419e-8135-4863748b58fa">


Release Notes:

- N/A

crates/collab_ui/src/chat_panel/message_editor.rs

@@ -306,6 +306,7 @@ impl MessageEditor {
                     server_id: LanguageServerId(0), // TODO: Make this optional or something?
                     lsp_completion: Default::default(), // TODO: Make this optional or something?
                     confirm: None,
+                    show_new_completions_on_confirm: false,
                 }
             })
             .collect()