v0.7.1

Fixed

• model_validate_json no longer crashes for Models that carry an opaque field. The null placeholder model_dump_json writes is dropped during JSON-payload conversion (the opaque translation's from_json would otherwise raise, by design); the field falls back to its declared default. A required opaque field with no default surfaces a clean missing_required ValidationError on round-trip instead of a bare TypeError. docs/guide/fields.md updated to spell out this behaviour precisely.

Full changelog: https://github.com/panproto/didactic/blob/main/CHANGELOG.md