Stop allow setting flat option

[jyu00]

What is the expected feature or enhancement?

The primitives today allow setting a nested option by itself. For example,

estimator.options.update(extrapolator="linear")

is allowed and would update the "correct" option in ZneOptions. However, this only works if all options have unique names, which is no longer true in V2 primitives. So setting something like num_randomizations could yield unexpected results.

Acceptance criteria

Only accept nested options in options.update().

[ihincks]

Here is a pattern for BaseModel

    def update(self, **fields):
        """Perform recursive update."""
        for key, value in fields.items():
            # Update nested field
            if isinstance(getattr(self, key, None), BaseModel):
                getattr(self, key).update(**value)
            else:
                setattr(self, key, value)

which could be adapted to our options with https://docs.pydantic.dev/latest/api/dataclasses/#pydantic.dataclasses.is_pydantic_dataclass

estimator.options.update(twirling={"enable_gates": True})