Create GCS folder markers as directories in GCSToSFTPOperator

[yuseok89]

related: #72638

Picks up #72638, closed under the open pull request limit. A GCS folder marker (folder/) was stored as an empty file at <dest>/folder, because os.path.normpath strips the slash, and the folder's children then failed to transfer.

Review feedback on #72638 and how it is addressed

• @aaron-y-chen: move_object left skipped markers in the bucket, and a later run with only folder/ failed. A marker is now created as a directory and then deleted when moving. A later run is a no-op, since SFTPHook.create_directory returns for an existing directory.
• @olegkachur-e: name/ always means a folder; prefer a switch over corner-case handling. Any name ending in / is created as a directory, never a file, and the earlier overlap and existing-directory checks are removed. The switch is the existing create_intermediate_dirs rather than a new parameter. With False, markers are skipped and left in the bucket.
• @olegkachur-e: bug fix or breaking change? Added a warning to the Google provider changelog, including that an empty file left by an earlier version must be removed. The classification is left to the release manager.

Tests: test_gcs_to_sftp.py passes; with the operator reverted to main, only the 4 new tests fail.

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  • Claude Code (Opus 5.5)

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[olegkachur-e]

LGTM, great work!👍

[aaron-y-chen]

Thanks, LGTM!