Return list of destination URIs from GCSToGCSOperator

[yuseok89]

Related: #11323

• Return value: execute() now returns a list[str] of destination URIs for copied objects (single and multiple files). This operator did not return a value before, so unwrap_single was not added.

• Deduplication for multi-file copy: When multiple source paths map to the same destination (e.g. source_objects=["src/foo.png", "src/data/"] with foo.png under data/), the same URI can appear more than once. The result list is deduplicated while preserving order via dict.fromkeys.

• Directory-like URIs excluded from return: If GCS list returns keys ending with / (e.g. prefix/), they are still copied, but such URIs are filtered out of the returned list so only file URIs are included.

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Test screenshot

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[shahar1]

Excellent work! LGTM.
Failing CI job seems unrelated - hopefully it should be fixed soon by another PR.
Please let me handle the rebase and merging, feel free to take on other issues.
I'll wait couple of days to give the chance for Google team to review.

CC: @VladaZakharova @MaksYermak