pysimlin: fix schema path (doc/ -> docs/) and add missing ModelGroup dataclass

[karlhillx]

Fixes #550 and fills the ModelGroup gap it revealed. Two changes:

1. Fix the schema path in test_json_types.py from doc/ to docs/; activates the 4 schema-compliance tests that have been silently skipped.

2. Add the missing ModelGroup dataclass and groups field to the Python Model class. Rust json::Model, TS JsonModel, and the JSON schema all have it; Python was the only layer missing it. groups is optional in the schema so this is backward-compatible; existing serialized data without groups still deserializes fine.