[DPE-10841] test(database): cover the manager's mapping caches and user map (5/7)

[marceloneppel]

Issue

PRs 1–4 moved postgresql_provider.py into the library. The charm-side tests that covered it are deleted by the adoption PRs, so the coverage has to exist here first.

Solution

Ports the charm provider tests onto the library manager, starting with its state half: the substrate-derived role name, the username and prefixed-database cache round trips, collect_user_relations and the user hash it derives, the peer-sync comparison, and plugin expansion.

The app-secret labels are asserted explicitly — renaming them would orphan the caches every deployed cluster already holds, and nothing else in the tree would notice.

Every test was checked by mutating the production code it targets and confirming it goes red; reverting the mutation restores green. Run with PYTHONDONTWRITEBYTECODE=1 and a __pycache__ sweep, since a byte-length-identical mutation otherwise leaves the stale bytecode valid.

Stacked on 4/7. Next: 6/7.

Checklist

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