Unit test TerminologyCacheAdapter backends

The contract test pattern (shared test function called for each backend) is the recommended approach — it avoids duplicating 8+ test cases for each backend and ensures consistent behavior. If TerminologyCacheAdapter wraps both SharedPreferences and Hive behind a single interface, define an abstract `CacheBackend` interface and test each concrete implementation independently. For JSON serialization, use `jsonEncode` / `jsonDecode` from `dart:convert`; do not use `toString()` on the map. Timestamp storage: serialize as `DateTime.toUtc().toIso8601String()` and deserialize with `DateTime.parse(...).toLocal()` — document the UTC convention in a code comment.

Ensure `setUp()` in each test group resets both the mock storage AND any in-memory state the adapter may hold.

Use `SharedPreferences.setMockInitialValues({})` in setUp() to reset state between tests. For Hive, use a fake in-memory box (implement a mock Box or use hive's own test utilities). Group tests by backend implementation. Write a shared behavior contract as a function `void runCacheAdapterContractTests(TerminologyCacheAdapter Function() factory)` and invoke it for each backend — this ensures both implementations satisfy the same contract.

Special character test: create a map with key `'norsk.æøå'` and value `'Norsk tekst: «hei» ny linje'` and assert round-trip fidelity. Timestamp test: capture DateTime.now() before write(), read back cachedAt, assert it is within 1 second of the captured time.