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/ecstore/src/config/com.rs

@@ -1355,7 +1355,7 @@ mod tests {
                 size: self.data.len() as i64,
                 actual_size: self.data.len() as i64,
                 is_dir: false,
-                user_defined: HashMap::new(),
+                user_defined: Arc::new(HashMap::new()),
                 parity_blocks: 0,
                 data_blocks: 0,
                 version_id: None,
@@ -1363,8 +1363,8 @@ mod tests {
                 transitioned_object: Default::default(),
                 restore_ongoing: false,
                 restore_expires: None,
-                user_tags: String::new(),
-                parts: Vec::new(),
+                user_tags: Arc::new(String::new()),
+                parts: Arc::new(Vec::new()),
                 is_latest: true,
                 content_type: Some("application/json".to_string()),
                 content_encoding: None,