Make _MiddlewareFactory compatible with Callable

[eltoder]

Summary

Make app a positional-only argument so that a Callable[[ASGIApp, ...], ASGIApp] can be passed in place of _MiddlewareFactory.

We can also consider making _MiddlewareFactory and moving it into starlette.types. This will make it easier to type arguments for add_middleware().

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.

[adriangb]

I find specifying positional only parameters in Protocols confusion, however I checked this case and I think it's correct:

import typing

class Plain(typing.Protocol):
    def __call__(self, x: int, y: str) -> None: ...

class Positional(typing.Protocol):
    def __call__(self, a: int, /, y: str) -> None: ...


def foo(x: int, y: str) -> None:
    ...


def bar(x: int, /, y: str) -> None:
    ...


a: Plain = foo  # ok
b: Plain = bar # err
c: Positional = foo  # ok
d: Positional = bar # ok

Passes on https://mypy-play.net/?mypy=1.4.1&python=3.13&gist=8972af53af6b9d67e6c255a5de32db18

[adriangb]

We can also consider making _MiddlewareFactory and moving it into starlette.types. This will make it easier to type arguments for add_middleware()

This part I did not understand

[eltoder]

@adriangb thank you for merging!

This part I did not understand

Sorry, missed a word:

We can also consider making _MiddlewareFactory public.

The rationale is that add_middleware() is a public function and occasionally I need to refer to the type of its argument. For example, I have a function that returns which middleware to use based on environment. I need to write its return type. With this PR I can write it as Callalble[], which is nice. But it may be even more clear and explicit to write it as MiddlewareFactory. However, currently the latter is not public. Here's a sketch of how this can be used: https://mypy-play.net/?mypy=1.13.0&python=3.13&gist=e0652be937c8cfc1d56a5d4dafa0bd89