* filer: auto clean empty s3 implicit folders
Explicitly tag implicitly created S3 folders (parent directories from object uploads) with 'Seaweed-X-Amz-Implicit-Dir'.
Update EmptyFolderCleaner to check for this attribute and cache the result efficiently.
* filer: correctly handle nil attributes in empty folder cleaner cache
* filer: refine implicit tagging logic
Prevent tagging buckets as implicit directories. Reduce code duplication.
* filer: safeguard GetEntryAttributes against nil entry and not found error
* filer: move ErrNotFound handling to EmptyFolderCleaner
* filer: add comment to explain level > 3 check for implicit directories
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