Decide on config surface for operator specs

[MacdonaldJoshuaCaleb]

The adapter currently accepts operators as a plain dict (default, tr, bdf2) and translates to RunConfig/OperatorSpecs in to_run_config() to keep the YAML/Pydantic schema simple and avoid arbitrary_types_allowed. Decide whether/how we should expose OperatorSpecs directly.

Options:

• Keep dict input + translation (status quo).
• Allow OperatorSpecs input and set arbitrary_types_allowed=True (or wrap OperatorSpecs to be Pydantic-friendly).

Document the mapping and rationale either way.