fix(@angular/build): ensure chokidar watcher is ready before returning #34122

When initializing chokidar.watch with ignoreInitial: true, files visited during the initial scan are treated as the initial baseline and do not emit change events. If createChokidarWatcher returns before the initial scan completes, subsequent file modifications made shortly after setup can be visited for the first time during the initial scan, causing the change event to be dropped.

Awaiting the ready event ensures the watcher's initial scan is completed and the watcher is actively listening before returning. This resolves test flakiness in rebuild error detection tests where files are modified immediately after the initial build.