Fix recursive stale DAG cleanup in S3 local sync

[siewcapital]

What this PR does

S3Hook.sync_to_local_dir(..., delete_stale=True) currently checks only the top-level entries of local_dir when deleting stale files. As a result, stale DAG files inside nested folders are never removed.

This PR makes stale cleanup recursive by traversing all descendants of local_dir (deepest path first), so stale nested files can be deleted and then their empty parent directories removed.

Why this change

This addresses #62622 where S3DagBundle keeps stale DAGs in subfolders because Path.iterdir() is not recursive.

Tests

Updated test_sync_to_local_dir_behaviour to cover nested stale cleanup:

• creates stale/nested/dag_stale.py locally (not in S3)
• verifies stale nested file is deleted
• verifies both stale/nested and stale empty directories are deleted

I also validated syntax compilation for the touched files locally.

Closes #62622

[boring-cyborg]

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