Skip GCS folder-marker keys in GCSToS3Operator

[yuseok89]

GCSToS3Operator copies every name GCSHook.list() returns, including the trailing-/ folder markers a GCS listing contains. They land in S3 as spurious empty keys and in the XCom output. With flatten_structure=True a marker has no basename, so it resolves to the destination prefix itself and a zero-byte object is written over it.

This applies the same overlap-aware rule S3ToGCSOperator uses (#65724): a trailing-/ key is dropped only when it is also a strict prefix of another key in the same listing, so a real object like lonely/ is kept. Flattening needs one addition — a kept lonely/ still flattens to an empty name, so keys with no basename are dropped as well. Skipped keys are logged, and the returned list is now sorted and de-duplicated.

related: #64966, #65724

Before / After

Verified end to end against a real GCS bucket and a real S3 bucket. Listing: src/ (a marker), src/file.txt, and lonely/ (a real object ending in /).

	S3 keys	XCom
Before	src/, src/file.txt, lonely/	all three
After	src/file.txt, lonely/	those two

With flatten_structure=True into s3://bucket/dest/:

	S3 keys	XCom
Before	dest/, dest/file.txt	all three
After	dest/file.txt	src/file.txt

dest/ is a zero-byte object on the destination prefix, because both markers flatten to an empty basename. lonely/ is dropped only under flattening, where it has no name left.

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