- Extract assert_hash_fields_consistent() helper that verifies presence
parity and value equality, replacing body.get()-based assertions that
treated missing keys and explicit nulls identically.
- Conftest seeded_asset fixture and seed-asset list assertions now check
key absence directly, so a regression that surfaces null fields would
be caught (validates exclude_none behavior).
- DRY duplicate hash expression in _build_asset_response.
- Add list-endpoint coverage asserting hash is present and consistent on
populated assets.
- Add schema-level test asserting AssetCreated inherits the hash field
from Asset, guarding against future inheritance drift.
