Create a bucket

Provisions a Khotan-managed storage bucket synchronously and responds 201 with the persisted record whether setup ends ready or failed; branch on the returned status. A bucket is a private folder subtree in your organization's file storage, reached through the Files API with a scoped key. Supply an idempotencyKey to make retries safe — a retry carrying the same key returns the original bucket instead of creating a second one; without a key, every call creates a new bucket.