[DPE-11045] test(logical-replication): cover the ported module (6/8)

[marceloneppel]

Issue

Sixth PR of the logical replication migration series (after #270; the restore-side guard follows in #338): the unit coverage for the ported module, split out of #270 so each PR stays under ~1000 changed lines.

Solution

• Ports the circular-replication coverage from the charms' fix PR onto the library test charms (tests/unit/test_logical_replication.py): chain building, subscriber and publisher circular checks, validation blocking, non-circular passing, and stale-error handling.
• Covers the publisher slot lifecycle and the transient-race create_subscription retry in tests/unit/test_postgresql.py: create on the publisher, deterministic drops with walsender-race retries.
• Adds the synchronous_mode_strict gating test in tests/unit/test_config_manager.py (strict mode only applied when cluster_initialised).
• Depends on [DPE-11045] fix(logical-replication): harden the ported module (5/8) #270's test-charm wiring (the two logical-replication relations in the test-charm metadata and the combined VM test charm) - the harness cannot add those relations without it.

Checklist

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