format=json loose mode gives valid JSON but no shape — models default
to emitting {} when the system prompt doesn't list fields. Prepend a
schema-guidance system message with the full Pydantic schema (after
the existing null-branch sanitiser) so the model sees exactly what
shape to produce. Pydantic still validates on parse.
Unit tests updated to check the schema message is prepended without
disturbing the caller's own messages.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
