chore: add Rapid x region sys test

[chandra-siri]

feat: Add system test for cross-region buckets

• Adds a new system test, test_basic_wrd_x_region, to verify functionality with cross-region GCS buckets.

• Also updates the Cloud Build configuration to pass the necessary _CROSS_REGION_BUCKET environment variable to the test environment

This PR is a cherry-picked from #1760 because that PR is blocked by b/489420625

[gemini-code-assist]

Summary of Changes

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This pull request introduces a new system test to validate the behavior of asynchronous multi-range downloads and appends with Google Cloud Storage buckets configured for cross-region access. It ensures that the system can correctly handle data operations across different geographical regions, and updates the build process to support this new testing scenario.

Highlights

• New System Test: A new system test, test_basic_wrd_x_region, has been added to verify functionality with cross-region GCS buckets, covering various object sizes.
• Cloud Build Configuration Update: The Cloud Build configuration (cloudbuild/zb-system-tests-cloudbuild.yaml) was updated to pass the _CROSS_REGION_BUCKET environment variable to the test environment, enabling cross-region tests.
• Retry Counter for AsyncMultiRangeDownloader: An _open_retries counter was introduced in AsyncMultiRangeDownloader to track the number of retry attempts when opening the underlying bidi-gRPC connection, and this counter is asserted in the new system test.

Changelog

• cloudbuild/zb-system-tests-cloudbuild.yaml
  • Passed CROSS_REGION_BUCKET environment variable to the test environment via gcloud compute ssh command.
• google/cloud/storage/asyncio/async_multi_range_downloader.py
  • Added _open_retries attribute to AsyncMultiRangeDownloader to count retry attempts.
  • Modified on_error callbacks within the open method to increment the _open_retries counter.
• tests/system/test_zonal.py
  • Declared _CROSS_REGION_BUCKET environment variable by retrieving it from os.getenv.
  • Added a new parameterized system test test_basic_wrd_x_region to test write, read, and delete operations with cross-region buckets.
  • Included assertions for _open_retries within the new cross-region test to verify retry behavior.

Activity

• No specific human activity (comments, reviews, etc.) has been recorded for this pull request yet.

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[gemini-code-assist]

Code Review

This pull request adds a system test for cross-region buckets, which is a valuable addition. My review focuses on improving the consistency of environment variable naming for better maintainability. I've suggested changes in both the Cloud Build configuration and the test file to align the new variable with existing conventions.