I like the idea of a centralized login function that makes some common patterns easy, but I think if we're centralizing this, we should go just a step further by:

Optionally allowing login of explicit users (handling LocalSite associations as needed).
Allowing an explicit LocalSite instance, to constrain tests less and to not always require the fixtures for those tests that are managing custom LocalSite instances.

Maybe this signature?

def login_user(
    self,
    *,
    admin: bool = False,
    local_site: (bool | LocalSite) = False,
    username: (str | None) = None,
) -> User:
    ...


Then we can continue to use it as-is, but can also specify an explicit username or LocalSite instance. That'll keep it more generally-useful without losing what made this convenient in API tests.
The implementation would have to do the "create a proper user" steps outlined in the docstring if a username or custom local_site is provided, of course. But it could keep existing logic so existing tests don't break.

This is newly-added here, not changed.

Can you mention how this can be a LocalSite too.

I think we'll also need to add doc as a user to the local site that gets passed in, because if someone's passing in a local site then it's likely a site that doesn't already include doc as a user of it.

Missing a trailing comma.

Ship It!