style: Use bot? predicate instead of role comparison

Changed `message.creator.role == "bot"` to `message.creator.bot?` per
Copilot code review. This is the idiomatic Rails pattern for enum checks
and consistent with how other role checks are done in the codebase.

Co-authored-by: openhands <openhands@all-hands.dev>

app/controllers/messages/by_bots_controller.rb

@@ -42,7 +42,7 @@ class Messages::ByBotsController < MessagesController
         creator: {
           id: message.creator.id,
           name: message.creator.name,
-          is_bot: message.creator.role == "bot"
+          is_bot: message.creator.bot?
         }
       }
     end