[DPE-10840] test(config): unit tests and golden fixtures for the update_config migration (5/6)

[marceloneppel]

Issue

The update_config migration's unit tests and patroni-template golden fixtures were bundled with the code changes, pushing each code PR in the stack past the review-size target.

Solution

Split the tests out of the code PRs so each code change lands small and self-contained, with the tests landing together here on top of the code PRs. Only the minimal edits to existing tests needed to keep each code PR green stayed with the code; every new test and both golden fixtures are here.

Covers the peer-state config/user hash accessors (#181), PostgreSQL parameter building and resource introspection (#182), the substrate-conditional patroni template and its per-substrate golden render matrix (#183), and the update_config orchestration (#180).

The golden matrix renders the lib template and the preserved pre-merge charm template with the same context and compares the parsed documents rather than the bytes, because the lib template emits one field order for both substrates instead of reproducing each charm's, and mapping order is not part of the configuration. Sequences still compare in order, so pg_hba — where PostgreSQL takes the first matching rule — and partner_addrs stay pinned.

One case sits outside that matrix by design: the K8s data paths are asserted to follow the workload's major version. The matrix pins the version at 16, where the parameterised and hardcoded spellings render identically, so it cannot see the difference the charm template carried — bin_dir followed the version while the WAL, PostgreSQL-log and Patroni-log paths did not.

Checklist

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