pandaproxy/rest: return table identity for pre-translation iceberg topics

The translation_state endpoint previously omitted Iceberg-enabled topics
that the coordinator had not yet started tracking (e.g. empty topics with
no translated records). This forced clients to guess the Iceberg namespace
and table name from catalog config. Return status=ENABLED with the
namespace, table, and DLQ table name derived from table_id_provider for
these topics, with empty partition states and no snapshot id.

The contract created by this change is that empty partition states and
no snapshot id indicates no committed translation (yet). It also rests
on the assumption that the namespace and table name can be computed from
the configuration, requiring no coordination with the catalog. This is
true now and should remain true in the future.