Flake in `run_storage_bidi_write` test

This flaked:

> **Commit:** [4680822](https://github.com/googleapis/google-cloud-rust/commit/46808228d2875e1e31c453ddc2929d721235b81f)
>
> ```
> failures:
>
> ---- storage::run_storage_bidi_write stdout ----
> cleaning up 0 buckets
> full error: the service reports an error with code INTERNAL described as: A zonal bucket can't currently be created in this zone. Try again later, or try creating the bucket in another zone.
>
> Caused by:
> code: 'Internal error', message: "A zonal bucket can't currently be created in this zone. Try again later, or try creating the bucket in another zone."
> because: Error { kind: Service(ServiceDetails { status_code: None, headers: Some({"pcx-high-bwd-bin": "KgSYAZEB", "grpc-server-stats-bin": "AADJp6oSAAAAAA", "endpoint-load-metrics-bin": "MXbYNzsyuFlASWlJtxOrVtw/", "content-type": "application/grpc", "grpc-accept-encoding": "identity, deflate, gzip", "x-guploader-uploadid": "AJjja9aBd4lQ5qpIKQLO7yiRxIEORiyNrJhhh26ZvyEnaBi_YzB0bKWWgQxAAGC3KYJKrcQ", "content-length": "0", "date": "Sat, 05 Sep 2026 10:50:58 GMT"}), status: Status { code: Internal, message: "A zonal bucket can't currently be created in this zone. Try again later, or try creating the bucket in another zone.", details: [] } }), source: Some(Status { code: Internal, message: "A zonal bucket can't currently be created in this zone. Try again later, or try creating the bucket in another zone.", metadata: MetadataMap { headers: {"pcx-high-bwd-bin": "KgSYAZEB", "grpc-server-stats-bin": "AADJp6oSAAAAAA", "endpoint-load-metrics-bin": "MXbYNzsyuFlASWlJtxOrVtw/", "content-type": "application/grpc", "grpc-accept-encoding": "identity, deflate, gzip", "x-guploader-uploadid": "AJjja9aBd4lQ5qpIKQLO7yiRxIEORiyNrJhhh26ZvyEnaBi_YzB0bKWWgQxAAGC3KYJKrcQ", "content-length": "0", "date": "Sat, 05 Sep 2026 10:50:58 GMT"} }, source: None }) }
> because: Status { code: Internal, message: "A zonal bucket can't currently be created in this zone. Try again later, or try creating the bucket in another zone.", metadata: MetadataMap { headers: {"pcx-high-bwd-bin": "KgSYAZEB", "grpc-server-stats-bin": "AADJp6oSAAAAAA", "endpoint-load-metrics-bin": "MXbYNzsyuFlASWlJtxOrVtw/", "content-type": "application/grpc", "grpc-accept-encoding": "identity, deflate, gzip", "x-guploader-uploadid": "AJjja9aBd4lQ5qpIKQLO7yiRxIEORiyNrJhhh26ZvyEnaBi_YzB0bKWWgQxAAGC3KYJKrcQ", "content-length": "0", "date": "Sat, 05 Sep 2026 10:50:58 GMT"} }, source: None }
> Error: the service reports an error with code INTERNAL described as: A zonal bucket can't currently be created in this zone. Try again later, or try creating the bucket in another zone.
>
> Caused by:
> code: 'Internal error', message: "A zonal bucket can't currently be created in this zone. Try again later, or try creating the bucket in another zone."
>
>
> failures:
> storage::run_storage_bidi_write
> ```

_Originally posted by @dbolduc in [#6670](https://github.com/googleapis/google-cloud-rust/issues/6670#issuecomment-5578693220)_