agent: Add a whitespace after inserting @-mention to allow for continuous typing (#30381)

Release Notes:

- agent: Added a space after @-mentioning something in the message
editor to allow for continuous typing.

---------

Co-authored-by: Peter Tripp <peter@zed.dev>
Co-authored-by: Danilo Leal <daniloleal09@gmail.com>

crates/agent/src/context_picker.rs

@@ -766,6 +766,7 @@ pub(crate) fn insert_crease_for_mention(
 
         let ids = editor.insert_creases(vec![crease.clone()], cx);
         editor.fold_creases(vec![crease], false, window, cx);
+
         Some(ids[0])
     })
 }