Investigation: Should fly_scan have a -> MsgGenerator[None] return annotation?

`fly_scan `is the only plan in the file missing a -> MsgGenerator[None] return type annotation, which is inconsistent and causes pyright to infer a broader type.

Investigate:

- Does omitting the annotation actually cause a pyright error currently, or does inference cover it?
- Are there any downstream consumers (e.g. BlueAPI introspection) that depend on the return type annotation being explicit?
- If yes to either: add the annotation. If pyright is already satisfied by inference and BlueAPI doesn't require it, document why it is intentionally omitted.

Start in the file containing the other plans and compare fly_scan with their return annotations. Run pyright to check whether inference reports an error, then search BlueAPI introspection consumers for dependencies on explicit annotations. Done means confirming the requirement and either adding the annotation or documenting why omission is intentional.

Written by the indexing model from the issue text.