Fix panic when re-editing old message with creases (#32017)

Co-authored-by: Cole Miller <m@cole-miller.net>

Release Notes:

- agent: Fixed a panic when re-editing old messages

---------

Co-authored-by: Cole Miller <m@cole-miller.net>
Co-authored-by: Cole Miller <cole@zed.dev>

crates/agent/src/thread.rs

@@ -1032,6 +1032,7 @@ impl Thread {
         id: MessageId,
         new_role: Role,
         new_segments: Vec<MessageSegment>,
+        creases: Vec<MessageCrease>,
         loaded_context: Option<LoadedContext>,
         checkpoint: Option<GitStoreCheckpoint>,
         cx: &mut Context<Self>,
@@ -1041,6 +1042,7 @@ impl Thread {
         };
         message.role = new_role;
         message.segments = new_segments;
+        message.creases = creases;
         if let Some(context) = loaded_context {
             message.loaded_context = context;
         }