fix: make create-replication action fail cleanly without async-replication relation

[SAY-5]

Issue

Closes #1675. Running juju run postgresql/<unit> create-replication name=default on a unit with no async-replication relation crashed with AttributeError: 'NoneType' object has no attribute 'name' because _on_create_replication dereferenced self._relation.name before checking that the relation existed.

Solution

Guard self._relation for None and call event.fail with a clear message instructing the operator to create the offer and relate the clusters first. Added a regression test exercising the missing-relation path.

Checklist

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

[taurus-forever]

Thank you for the contribution!

At least unit tests are unhappy here:

tests/unit/test_async_replication.py::test__configure_standby_cluster[True] FAILED
tests/unit/test_async_replication.py::test__configure_standby_cluster[False] FAILED
tests/unit/test_async_replication.py::test_handle_replication_change[True] FAILED
tests/unit/test_async_replication.py::test_handle_replication_change[False] FAILED
tests/unit/test_async_replication.py::test_handle_forceful_promotion[True] FAILED
tests/unit/test_async_replication.py::test_handle_forceful_promotion[False] FAILED

Please adopt. Please also sign commits and SLA agreement (new gmail email used?).

[SAY-5]

Fixed — the regression test set type(relation)._relation = PropertyMock(...) directly, which mutated the class globally and leaked into later tests. Wrapped it in a patch.object context manager so the descriptor is restored on exit. All 32 tests in test_async_replication.py pass locally now. CI should re-run.

[marceloneppel]

Thanks for the fixes, @SAY-5! Did you check the CLA?

[SAY-5]

Yes — CLA signing is on my checklist. I'll get that signed shortly so this can land.