Tidy up adapter in the scan_nd plan

[noemifrisina]

Current Behavior

In the scan_nd plan, it is not obvious how the adapter function works when _verify_1d_step is True.
It appears to be redefining per_step to be usable for the 1d case since the signatures of the one_nd_step and one_1d_step are slightly different. However the arguments passed to the adapter do not match the ones passed to user_per_step when doing this operation.

            user_per_step = per_step

            def adapter(detectors, step, pos_cache):
                # one_nd_step 'step' parameter is a dict; one_id_step 'step'
                # parameter is a value
                (step,) = step.values()
                return (yield from user_per_step(detectors, motor, step))

            per_step = adapter  # type: ignore

Context

The issue came up while adding type hints to plans in #1712 , which showed a mismatch between the signatures of expression and variable:

src/bluesky/plans.py:1148: error: Incompatible types in assignment (expression has type "Callable[[Any, Any, Any], Any]", variable has type "Callable[[Sequence[Readable], Movable, Any, Callable[[Sequence[Readable]], Generator[Msg, Any, Mapping[str, Reading]]] | None], Generator[Msg, Any, Any]] | Callable[[Sequence[Readable], Mapping[Movable, Any], dict[Movable, Any], Callable[[Sequence[Readable]], Generator[Msg, Any, Mapping[str, Reading]]] | None], Generator[Msg, Any, Any]] | None")  [assignment]

Temporary workaround was to #type: ignore it.