Fix ThreadSanitizer data races between bundle processing and async background writer closing in FileBasedSink and WriteFiles

[stankiewicz]

Fixes ThreadSanitizer data race errors reported during asynchronous writer closing in WriteFiles and FileBasedSink.

Unsynchronized Cross-Thread Lifecycle Access:
In WriteFiles.WriteShardsIntoTempFilesFn, a FileBasedSink.Writer is instantiated, opened (writer.open()), and written to on the bundle processing thread. Once bundle processing completes, closeWriterInBackground(writer) submits asynchronous background tasks (MoreFutures.runAsync) to close each writer on a ForkJoinPool thread. Because plain Java fields (id, destination, outputFile, channel) lack release-acquire barriers across threads, ThreadSanitizer flags reads inside writer.close() as racing with writes inside writer.open().

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