fix(bucket): propagate bucket deletion to peer metadata caches (backlog#646) (#4326)

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.

crates/ecstore/src/bucket/metadata_sys.rs

@@ -74,6 +74,17 @@ pub async fn set_bucket_metadata(bucket: String, bm: BucketMetadata) -> Result<(
     Ok(())
 }
 
+/// Drop a bucket's cached metadata from the in-memory map.
+///
+/// This is the counterpart to [`set_bucket_metadata`] and is invoked when a
+/// bucket is deleted so peers stop serving stale cached configuration for it.
+/// Returns `true` if an entry was present.
+pub async fn remove_bucket_metadata(bucket: &str) -> Result<bool> {
+    let sys = get_bucket_metadata_sys()?;
+    let lock = sys.read().await;
+    Ok(lock.remove(bucket).await)
+}
+
 pub async fn get(bucket: &str) -> Result<Arc<BucketMetadata>> {
     let sys = get_bucket_metadata_sys()?;
     let lock = sys.read().await;
@@ -360,6 +371,17 @@ impl BucketMetadataSys {
         }
     }
 
+    /// Remove a bucket's cached metadata from the in-memory map.
+    ///
+    /// Returns `true` if an entry was present. Reserved meta buckets are ignored.
+    pub async fn remove(&self, bucket: &str) -> bool {
+        if is_meta_bucketname(bucket) {
+            return false;
+        }
+        let mut map = self.metadata_map.write().await;
+        map.remove(bucket).is_some()
+    }
+
     async fn _reset(&mut self) {
         let mut map = self.metadata_map.write().await;
         map.clear();