Fix Publication for partitioned tables

[raguidu]

Currently there is a bug in handling Postgres Publications for partitioned tables.

When a partitioned table is added to a publication, Postgres internally tracks only the child partitions in pg_publication_tables, not the parent table itself. This causes an issue in Terraform’s postgresql_publication resource: even though the parent table was added, a subsequent Terraform plan sees only the child partitions and assumes the parent table is missing. It then attempts to re-add the parent table, resulting in an error like:

pq: relation "my_partitioned_table" is already member of publication "my_publication"

This makes it impossible to manage publications declaratively for partitioned tables using Terraform without running into persistent apply errors.

What this PR does:

This PR updates the postgresql_publication resource logic to:

• Detect when child partitions listed in pg_publication_tables belong to a common parent table (via pg_inherits and pg_class)
• Report the parent table (e.g. public.my_partitioned_table) in place of individual partitions, ensuring the Terraform state matches the declared configuration
• Preserve behaviour for non-partitioned tables, avoiding any regressions

Compatibility:

Although not tested in all versions, this fix should be compatible with Postgres 10+ (which is the same compatibility of the current query) since I'm only adding old/established catalog views (pg_class, pg_inherits, pg_namespace).