Add `Operator` Pydantic model

[TimothyWillard]

op_system/src/op_system/specs.py

Lines 593 to 623 in bb41b22

	def _normalize_operators(
	raw_ops: object,
	*,
	axis_names: set[str],
	) -> list[dict[str, Any]]:
	"""Normalize operator metadata and validate axis references.
	Returns:
	List of normalized operator definitions.
	"""
	if raw_ops is None:
	return []
	if not isinstance(raw_ops, list):
	_raise_invalid_rhs_spec(detail="operators must be a list")
	out: list[dict[str, Any]] = []
	for idx, op in enumerate(raw_ops):
	op_map = _ensure_mapping(op, name=f"operators[{idx}]")
	axis_name = op_map.get("axis")
	if not isinstance(axis_name, str) or not axis_name.strip():
	_raise_invalid_rhs_spec(
	detail=f"operators[{idx}].axis must be a non-empty string"
	)
	axis_name_s = axis_name.strip()
	if axis_name_s not in axis_names:
	_raise_invalid_rhs_spec(
	detail=f"operators[{idx}] references unknown axis {axis_name_s!r}"
	)
	op_out = dict(op_map)
	op_out["axis"] = axis_name_s
	out.append(op_out)
	return out

Not clear exactly what these are. Other references:

op_system/README.md

Line 74 in bb41b22

- Operators: `operators` metadata is normalized and preserved in `NormalizedRhs.meta["operators"]`; wiring into solvers is future-facing (not yet consumed by op_engine).

op_system/tests/op_system/test_op_system_specs.py

Lines 89 to 103 in bb41b22

	def test_normalize_rhs_preserves_reserved_blocks_in_meta() -> None:
	"""Test that reserved future blocks are preserved in meta."""
	spec = {
	"kind": "transitions",
	"state": ["S", "I", "R"],
	"aliases": {"N": "S + I + R"},
	"transitions": [{"from": "S", "to": "I", "rate": "beta * I / N"}],
	"operators": {"default": {"scheme": "cn"}}, # reserved for future
	"sources": {"S": "0.0"}, # reserved for future
	}
	out = normalize_rhs(spec)
	assert out.kind == "transitions"
	assert out.meta.get("operators") == {"default": {"scheme": "cn"}}
	assert out.meta.get("sources") == {"S": "0.0"}