Use metadata-free GCS bucket references

[kmontemayor2-sc]

Summary

Replace object-only storage.Client.get_bucket(...) calls in GcsUtils with storage.Client.bucket(...).

These paths only construct blob references, list object prefixes, or delete objects. They do not read bucket metadata, so Client.bucket() is
behavior-preserving and avoids requiring storage.buckets.get for callers that only have object-scoped IAM roles such as roles/ storage.objectUser.

Changes

• Updated does_gcs_file_exist, delete_gcs_file_if_exist, count_blobs_in_gcs_path, delete_files, and _delete_files_in_bucket_dir to
  use metadata-free bucket references.
• Added unit coverage that fails if get_bucket() is used for these object-only helpers.
• Added integration coverage for real GCS object existence, counting, single-object deletion, batch deletion, and directory deletion.

Tests

• make unit_test_py PY_TEST_FILES="gcs_test.py": passed
• make integration_test PY_TEST_FILES="gcs_test.py": passed
• make type_check: passed
• uv run ruff check --config pyproject.toml gigl/common/utils/gcs.py tests/unit/utils/gcs_test.py tests/integration/common/gcs_test.py:
  passed
• uv run ruff format --check --config pyproject.toml gigl/common/utils/gcs.py tests/unit/utils/gcs_test.py tests/integration/common/ gcs_test.py: passed

Note: full make check_format passed Python/C++/Scala phases but failed on unrelated pre-existing Markdown formatting issues outside this
change.

[kmontemayor2-sc]

/all_test

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