The peer `DeleteBucketMetadata` RPC handler was a stub that returned
success without doing anything, and the delete-bucket flow never sent
the notification in the first place. As a result, after a bucket was
deleted other nodes kept serving its stale cached metadata.
Wire the whole path end to end:
- ecstore: add an in-memory `remove_bucket_metadata` (free fn) and
`BucketMetadataSys::remove`, the counterpart to `set_bucket_metadata`,
and export it through the `api::bucket::metadata_sys` facade.
- node_service: `handle_delete_bucket_metadata` now validates the bucket
name and actually drops the cached metadata for it.
- bucket_usecase: after a successful delete_bucket, notify peers via
`notification_sys.delete_bucket_metadata` in the background, symmetric
to the existing `notify_bucket_metadata_reload` path.
Also update the delete-bucket-metadata unit test to assert the
empty-bucket rejection instead of the old always-success stub, and drop
an unused `tracing::debug` test import left over from #4322.
Verified: cargo fmt; cargo check -p rustfs-ecstore; cargo test -p rustfs
--lib --features rio-v2 test_delete_bucket_metadata_empty_bucket; arch
guardrail scripts pass.
