Compute tests::usage_report_samples flaked

The integration test `tests::usage_report_samples` in `guide/samples/tests/compute.rs` flaked with a 503 SERVICE UNAVAILABLE error from Compute Engine while waiting for the long-running operation to disable the usage export bucket.

See https://pantheon.corp.google.com/cloud-build/builds;region=us-central1/a7ef48f8-5883-4ec4-ab1c-147e84bba83d?project=rust-sdk-testing&e=13803378&mods=allow_workbench_image_override

Start with the integration test tests::usage_report_samples in guide/samples/tests/compute.rs and reproduce or inspect the reported Cloud Build failure. Trace the wait for the long-running operation that disables the usage export bucket and determine how the 503 SERVICE UNAVAILABLE failure should be handled. Done means the test no longer flakes while still detecting genuine failures.

Written by the indexing model from the issue text.