Add GCSToAzureBlobStorageOperator for GCS to Azure Blob transfer

[yuseok89]

Adds GCSToAzureBlobStorageOperator to copy objects from Google Cloud Storage to Azure Blob Storage.
There was no existing operator for this path (GCS → Azure Blob), so this fills that gap. The implementation follows the same overall patterns as S3ToAzureBlobStorageOperator.

Test Screenshots

Source files (GCS)

Default structure

Flatten structure

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[potiuk]

@yuseok89 Converting to draft — this PR doesn't yet meet our Pull Request quality criteria.

• ❌ Merge conflicts: this branch has conflicts with main and cannot be merged as-is. Rebase onto the current main to resolve: git fetch upstream main && git rebase upstream/main, resolve, force-push.

See the linked criteria for how to fix each item, then mark the PR "Ready for review". This is not a rejection — just an invitation to bring the PR up to standard. No rush.

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[potiuk]

Re-reading after the local rebase and noticed a few correctness issues worth a fixup before merge. Inline notes follow — items 1, 4, 6, 7 are real bugs (1 and 6 are the only ones I'd consider blocking), the rest are style/coverage. Items 1 and 4 have one-click suggestions; 2, 3, 5, 6, 7 are discussion comments because the fix touches non-contiguous lines.

Nice operator overall — much cleaner shape than the sibling S3ToAzureBlobStorageOperator (separate flatten_structure / keep_directory_structure semantics, OpenLineage facets, optional-dep handling via AirflowOptionalProviderFeatureException). The four issues below are mostly polish.

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[yuseok89]

@potiuk
Thanks again for the thorough review.
Every point landed and I learned a lot. The Windows path issue and the retry-mutation bug in particular were not on my radar. I'll be more careful with these patterns next time and think about how to catch them earlier on my side before the PR.

[github-actions]

uv.lock on main just moved via #66631 ("[main] Upgrade important CI environment"), commit 1aff9b6 and this PR currently conflicts.

Quickest fix:

git fetch upstream main && git rebase upstream/main
rm uv.lock && uv lock
git add uv.lock && git rebase --continue
git push --force-with-lease

Automated nudge — ignore if you're not ready to rebase. This comment is updated in place on future uv.lock bumps.