ZIm/crates/eval/examples/virtio_block_request_refactor/criteria.md

1. The changes remove unnecessary generic type parameters from the FileEngine, AsyncFileEngine, and related structures by directly using the PendingRequest type, simplifying type signatures and improving code clarity.
2. Error handling is unified through the replacement of UserDataError with RequestError that specifically carries PendingRequest information, ensuring consistent error propagation with request context.
3. The WrappedUserData struct is renamed to WrappedRequest and directly embeds PendingRequest, aligning terminology with the virtio block device’s request lifecycle and improving traceability.
4. Test code is updated to use PendingRequest::default() instead of placeholder () types, ensuring type consistency and proper request initialization in all scenarios.
5. Code organization is improved by consolidating imports (e.g., merging IO_URING_NUM_ENTRIES and PendingRequest imports) and removing redundant type parameters across async/sync I/O implementations.