Fix S3ToGCSOperator deferrable mode to return list of copied files

[yuseok89]

This PR addresses functionality originally proposed in #49768. Since that PR has been stalled, and #11323 is effectively blocked by it, I've proceeded with this implementation in the hope of unblocking progress.

If the original author of #49768 returns and continues his work, I can close this PR so that his proceeds instead. If not, would it be acceptable to merge this PR so that deferrable S3→GCS transfers can return the list of copied files and downstream tasks can consume them via XCom?

S3ToGCSOperator in deferrable mode now returns the list of copied file paths (same as non-deferrable mode), so downstream tasks can consume them via XCom. This fixes the previous inconsistency where deferrable=True did not return any value.

Verified in testing: both deferrable=True and deferrable=False return file names, not URIs. Returning URIs instead could be addressed in a separate PR if desired.

Screenshots

Deferrable state

XCom

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[SameerMesiah97]

Looks good. Just one nit if you want to implement it.

[potiuk]

@yuseok89 This PR has a few issues that need to be addressed before it can be reviewed — please see our Pull Request quality criteria.

Issues found:

• ⚠️ Testing requirements: The two new tests (test_execute_complete_success_returns_copied_files and the updated test_execute_complete_success) both pass context=mock.MagicMock() without a spec or autospec. Per the code review checklist, unspecced mocks silently accept any attribute access and can hide real bugs. The context mock is inherited from the pre-existing test pattern, but the new test also follows the same unspecced pattern.

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[yuseok89]

@potiuk
Thanks for the review. I initially kept MagicMock for consistency with the pre-existing test pattern, but I've updated the related execute_complete tests to use context={} instead. I've also rebased onto main as suggested.