fix: return 503 on lock contention instead of 500 (#3274)

* fix(ecstore): skip hidden metadata in walk limit

* fix(ecstore): match walk limit to visible versions

* fix(ecstore): avoid decoding all versions for limit

* fix: return 503 on lock contention instead of 500

When concurrent PUTs to the same key contend for the namespace write
lock, lock timeout and conflict errors were wrapped as
StorageError::other(...) → StorageError::Io(...), which fell through
to S3ErrorCode::InternalError (500) in the error mapping.

Only QuorumNotReached was correctly mapped to ServiceUnavailable (503);
all other lock errors (Timeout, AlreadyLocked, etc.) became 500.

Fix: map_namespace_lock_error now returns StorageError::Lock(err) for
non-quorum lock errors, and StorageError::Lock is mapped to
S3ErrorCode::ServiceUnavailable in the HTTP error conversion.

Affected paths:
- crates/ecstore/src/set_disk/lock.rs
- crates/ecstore/src/store/object.rs
- crates/ecstore/src/store/bucket.rs (make_bucket, delete_bucket)
- rustfs/src/app/object_usecase.rs (copy_object)

Regression test: test_concurrent_put_same_key_never_returns_500

* test: fail on unexpected lock contention errors

---------

Co-authored-by: houseme <housemecn@gmail.com>

crates/ecstore/src/set_disk/lock.rs

@@ -67,10 +67,7 @@ impl SetDisks {
                     achieved,
                 }
             }
-            other => StorageError::other(format!(
-                "Failed to acquire {mode} lock: {}",
-                self.format_lock_error_from_error(bucket, object, mode, &other)
-            )),
+            other => StorageError::Lock(other),
         }
     }
 

crates/ecstore/src/store/bucket.rs

@@ -81,7 +81,7 @@ impl ECStore {
                                 achieved,
                             }
                         }
-                        other => StorageError::other(format!("make_bucket: failed to acquire write lock on {bucket}: {other}")),
+                        other => StorageError::Lock(other),
                     })?,
             )
         } else {
@@ -185,7 +185,7 @@ impl ECStore {
                                 achieved,
                             }
                         }
-                        other => StorageError::other(format!("delete_bucket: failed to acquire write lock on {bucket}: {other}")),
+                        other => StorageError::Lock(other),
                     })?,
             )
         } else {

crates/ecstore/src/store/object.rs

@@ -243,7 +243,7 @@ impl ECStore {
                 required,
                 achieved,
             },
-            other => StorageError::other(format!("Failed to acquire {mode} lock on {bucket}/{object}: {other}")),
+            other => StorageError::Lock(other),
         }
     }