Add stale GCS test bucket cleaner - #40386

Adds cleanup for GCS buckets leaked by GcsUtil integration tests.

Before:

• Tests created randomized apache-beam-temp-bucket-<UUID> buckets.
• A failed or interrupted teardown could leave those buckets indefinitely.
• The scheduled cleanup workflow did not handle these buckets.

After:

• Lists buckets only from apache-beam-testing using the dedicated prefix.
• Revalidates the complete apache-beam-temp-bucket-<UUIDv4> name before deletion.
• Deletes only buckets older than 24 hours.
• Defaults to dry-run; the scheduled Gradle task explicitly passes --delete.
• Removes objects from leaked buckets before deleting them.
• Continues processing after individual failures and reports failures at the end.
• Runs the regression tests before the scheduled cleanup task.

Validation:

• python3 -m unittest test_stale_cleaner.py test_stale_gcs_buckets_cleaner.py — 29 tests passed.
• ./gradlew :beam-test-tools:testStaleGcsBucketsCleaner — passed.
• ./gradlew :beam-test-tools:removeStaleGcsBuckets --dry-run — Gradle wiring validated.
• Python compilation and git diff --check passed.
• Independent correctness and security reviews found no remaining P0, P1, or P2 issues.

CHANGES.md is not updated because this only changes internal test infrastructure.

Fixes #40347