fix(ecstore): optimize ObjectInfo clone and fix critical TODOs (#3149)

* fix(ecstore): optimize ObjectInfo clone and fix critical TODOs

## ObjectInfo hot-path clone optimization (issue #653 item 2)
- Wrap user_defined (HashMap), user_tags (String), parts (Vec) in Arc
- Clone cost reduced from ~20 heap allocations to O(1) ref count bump
- Updated 11 downstream access sites with explicit deref where needed

## Critical TODO/FIXME fixes (issue #653 item 7)
- rpc/peer_s3_client.rs: descriptive error message for empty peer response
- set_disk/write.rs: concurrent rollback deletes via tokio::spawn + join_all
- store_list_objects.rs: resolved FIXME with explanation + 4 regression tests
- store/bucket.rs: namespace write locks for make_bucket and delete_bucket

Skipped TODOs (too risky without broader context):
- multipart.rs:30 nslock — causes lock timeout in existing tests
- bucket.rs:88 cached list_bucket — needs cache invalidation strategy
- bucket.rs:149 replication delete — needs replication subsystem integration

1134 tests pass (1 flaky under parallel execution, passes individually).

* fix: address PR #3149 review comments

- Preserve NamespaceLockQuorumUnavailable variant in bucket create/delete
- Fix cancellation test to actually test cancellation (keep senders open)
- Update ObjectInfo consumers in app/ for Arc-backed fields
- Update bucket_usecase.rs user_tags to Arc<String>

* fix: update ObjectInfo Arc consumers

* fix: address ecstore merge review comments

* fix: cancel blocked merge output sends

* fix(ecstore): satisfy ObjectInfo clone clippy

crates/protocols/src/swift/object.rs

@@ -867,7 +867,7 @@ pub async fn copy_object(
 
     // 10. Prepare metadata for destination object
     // Start with source metadata
-    let mut new_metadata = src_info.user_defined.clone();
+    let mut new_metadata = (*src_info.user_defined).clone();
 
     // 11. If custom metadata headers provided, use those instead (Swift behavior)
     let mut has_custom_meta = false;