test(observers): cover the observer handlers (5/6)

[marceloneppel]

Issue

Part of the observers module migration from the PostgreSQL VM and K8s charms' 16/edge branches into this library. Stacks on #261.

Solution

Ports the cluster topology observer's start/stop and raft-observer unit-rendering suites from the VM charm onto the library harness: patch targets move from the charms' _peers/_peer_members_ips properties to the library state (peer_relation/peer_members_ips), the harness peer relation backs the positive cases, and the raft-observer test reads its expected systemd unit content from the package templates instead of the charm's templates/ directory.

Adds the authorisation rules observer's start/stop coverage (the K8s charm had none): guards on status, peers, container connectivity and cluster initialisation, the already-running and dead-pid restart paths, SIGTERM stop with databag cleanup, and the substrate gating of both handlers' construction on the abstract charm.

Every test runs under both substrates via the session fixture and skips itself on the substrate that does not ship the handler.

Provenance: tests ported from tests/unit/test_cluster_topology_observer.py @ 03494fb7b77f96b3108d5968b5eb4d4b61fb6f7f (postgresql-operator 16/edge); the authorisation rules handler tests are new, matching the ported behavior.

Checklist

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

Wiring up at canonical/postgresql-operator#1940 and canonical/postgresql-k8s-operator#1725.