[DPE-7882] fix(logical-replication): track replication chains and guard the publisher (3/8)

[marceloneppel]

Issue

Third PR of the logical replication migration series (after #267): the publisher-side guard for cyclic replication, part of fixing canonical/postgresql-operator#1085 and canonical/postgresql-k8s-operator#1052.

Solution

• Publications now carry replication-chains metadata recording the app path a table's data took (built from our own subscription's chains, extended when republishing).
• The offer side refuses to publish a table it is subscribed to from the requester: direct A <-> B loops on the same table and multi-hop A -> B -> C -> A loops both surface as publisher errors on the relation, which the subscriber side (next PR) turns into a blocked status.
• The create-publication branch moves into _create_new_publication to keep process_offer under the complexity budget.
• The subscriber-side circular helpers (_check_subscriber_circular_replication, _would_create_circular_replication) are introduced here unused; the next slice wires them into the subscriber's validation.

Checklist

• I have added or updated any relevant documentation.
• I have cleaned any remaining cloud resources from my accounts.