fix(data-usage): make thin usage-cache types a read-only projection (#5981)

Delete the dead ecstore save_data_usage_cache and the thin
DataUsageCache::marshal_msg it was the only caller of, drop the
Serialize derive (and the dead DataUsageCacheStorage trait with its
save path) from the thin projection types so no write path can exist
outside the scanner's canonical map-encoded writer, and pin the
persisted .usage-cache.bin wire bytes with cross-crate fixture tests
on both the scanner writer and the thin reader.

Refs rustfs/backlog#1828 (T1-T3).

crates/ecstore/src/data_usage/mod.rs

@@ -2150,30 +2150,6 @@ pub async fn load_data_usage_cache(store: &crate::set_disk::SetDisks, name: &str
     Ok(d)
 }
 
-#[instrument(skip(cache))]
-pub async fn save_data_usage_cache(cache: &DataUsageCache, name: &str) -> crate::error::Result<()> {
-    use crate::config::com::save_config;
-    use crate::disk::BUCKET_META_PREFIX;
-    use std::path::Path;
-
-    let Some(store) = runtime_sources::object_store_handle() else {
-        return Err(Error::other("errServerNotInitialized"));
-    };
-    let buf = cache.marshal_msg().map_err(Error::other)?;
-    let buf_clone = buf.clone();
-
-    let store_clone = store.clone();
-
-    let name = Path::new(BUCKET_META_PREFIX).join(name).to_string_lossy().to_string();
-
-    let name_clone = name.clone();
-    tokio::spawn(async move {
-        let _ = save_config(store_clone, &format!("{}{}", name_clone, ".bkp"), buf_clone).await;
-    });
-    save_config(store, &name, buf).await?;
-    Ok(())
-}
-
 /// Persist the current in-memory compression total to the backend.
 /// Resets the debounce counter so the next auto-persist won't fire
 /// immediately after this manual flush (intended for shutdown paths).