Stubs: flatten all service field sections, annotate multiple selectors as lists

[jeyrb]

I encountered this when trying to make sure Supernotify worked well with Pyscript.

It has its own notify action now, which is much richer than the plain vanilla notify.send_message style to be easier to use from the actions UI. So a more complicated services.yaml, though other integrations might similarly be pushing what the configflow UI can do.

Problem

pyscript.generate_stubs gets the signature wrong for services whose services.yaml uses sections or multiple selectors.

• Sections. fix stub generator dropping additional_fields entries #867 flattened the sections named additional_fields and advanced_fields, but a section can have any name. Any other section becomes a bogus untyped parameter, and the fields inside it are dropped.
• multiple: true. A text, select or entity selector with multiple: true takes a list, but is annotated as a single value, so passing a list is reported as a type error.

For example, a service with a multimedia section and a multiple text field currently generates:

def notify(*, message: str | None=None, delivery: str | None=None, multimedia=None): ...

Change

• A field is treated as a section when it has fields and no selector, whatever its name.
• A selector with multiple: true is annotated as list[...] of its usual type.

The same service now generates:

def notify(*, message: str | None=None, delivery: list[str] | None=None, camera_entity_id: str | None=None, snapshot_url: str | None=None): ...

Testing

Added test_stubs_flatten_named_sections_and_multiple, which fails on master and passes with this change. The existing stub tests still pass.