Output message in console when error is resolved

enhance dev/sandbox feature request: currently, when sandbox is running, it produces no output unless there is a build error. this is probably smart. however i've noticed that after i fix the build error, since there is no output, every time i tab back into my sandbox terminal, the only thing i see is an error message, and it leaves me wondering what state the sandbox is in. (is it errored? is it now building correctly?). it would be nice, from my perspective, for a message to be printed out in the case where we switch from a broken state to a successful server state. Something like [checkmark] App successfully rebuilt, maybe. it would give me peace of mind when i go back to see the last known state explicitly.

まず、dev/sandbox の rebuild ステータス処理とコンソール出力の経路を追跡します。ビルドエラーを再現して修正し、サーバーが成功状態へ遷移したときに明示的な成功メッセージが出力され、既存のエラー出力がそのまま維持されることを確認します。

索引モデルが issue の本文から書いたものです。