Fix race condition when deleting labels (#4944)

Concurrent requests updating the same resource's labels could fail with
Sequel::NoExistingObject when both try to delete the same label. This
occurred because find().destroy loads the model first, then deletes by
ID - if another request deletes it in between, the delete affects 0 rows
and Sequel raises an error.

Changed to use where().destroy which deletes directly via SQL without
loading the model, making it atomic and idempotent.

Also removed unnecessary .try(:destroy) from annotations_update since
where() always returns a dataset, never nil.

Observed error:
CF-NoExistingObject: Attempt to delete object did not result in a single
row modification (Rows Deleted: 0, SQL: DELETE FROM
"service_instance_labels" WHERE ("id" = ...))

Author: philippthun

app/actions/annotations_update.rb

@@ -10,7 +10,7 @@ def update(resource, annotations, annotation_klass, destroy_nil: true)
           prefix, key_name = VCAP::CloudController::MetadataHelpers.extract_prefix(key)
 
           if value.nil? && destroy_nil # Delete Annotation
-            annotation_klass.where(resource_guid: resource.guid, key_name: key_name).where(Sequel.or([[:key_prefix, prefix], [:key_prefix, prefix.to_s]])).try(:destroy)
+            annotation_klass.where(resource_guid: resource.guid, key_name: key_name).where(Sequel.or([[:key_prefix, prefix], [:key_prefix, prefix.to_s]])).destroy
             next
           end
 

app/actions/labels_update.rb

@@ -12,7 +12,7 @@ def update(resource, labels, label_klass, destroy_nil: true)
           prefix, name = VCAP::CloudController::MetadataHelpers.extract_prefix(label_key)
 
           if label_value.nil? && destroy_nil # Delete Label
-            label_klass.find(key_prefix: prefix.to_s, resource_guid: resource.guid, key_name: name)&.destroy
+            label_klass.where(key_prefix: prefix.to_s, resource_guid: resource.guid, key_name: name).destroy
             next
           end