ontology: `spec.py` Pydantic models reject spec-conformant documents carrying `datatype`

[christianeu-db]

Summary

converters/ontology/src/ossie_ontology/spec.py DatasetField and Metric omit datatype, while the base OssieObject sets extra="forbid". Any conformant document with datatype (including examples/tpcds_semantic_model.yaml) fails to parse with extra_forbidden.

Root cause

extra="forbid" + no datatype field on DatasetField/Metric, though the core spec defines datatype on both.

Repro (real converter, HEAD ab8fc1a — EXECUTED)

SemanticModel(**doc) with a field datatype: Integer:

1 validation error for SemanticModel
datasets.0.fields.0.datatype
  Extra inputs are not permitted [type=extra_forbidden, input_value='Integer', input_type=str]

Potential fix

Add datatype: str | None = None to DatasetField and Metric. This should be a non-breaking change. Value-enum validation is a separate, optional concern.