fix: Allow to inject async functions into functions wrapped with asynccontextmanager

[Zhenay]

When you try to inject synchronous or asynchronous dependencies into an asynchronous generator wrapped with contextlib.asynccontextmanager like this:

async def some_dep() -> int:
    return 5

@asynccontextmanager
@inject
async def some_async_gen(value: Annotated[int, Depends(some_dep)]):
    yield value

you see the error AttributeError: 'solve_async_gen' object has no attribute '_iter'.

The reverse order of the decorators works for sync dependincies only and raises the error AssertionError: You cannot use async dependency `some_dep` at sync main for async ones.

This pull request allows the inject function be used in such context with the direct order of the decorators. I don't see any possibility to provide the ability to use the reverse order because of the way asynccontextmanager is implemented in the standard library:

# contextlib.py - python 3.12
def asynccontextmanager(func):
    @wraps(func)
    def helper(*args, **kwds):
        return _AsyncGeneratorContextManager(func, args, kwds)
    return helper

As you can see, the decorator returns the synchronous wrapper, so when fast_depends execute build_call_model it determines the function as synchronous.

[Lancetnik]

Please, remove default value here and in sync test to make the test much representable

[Zhenay]

Done

[Lancetnik]

Thank you for the PRs a lot! Btw, what are u doing with FastDepends to find such bugs? 😅

[Zhenay]

I do nothing 😅
A collegue of mine created a very minimal DI system for one of our internal projects, but when I showed him FastDepends, he decided that it covers all his needs and he can replace self-written DI with FastDepends.
This internal projects uses async generators very widely, so the issue with injecting async depenedencies into async generators was discovered very quickly.
As far as I know, he have not used asynccontextmanagers with async dependencies yet, but I checked this scenario too as one of the most popular uses of async generators and discovered this issue

[Zhenay]

And thank you for your awesome projects. I use both FastDepends and FastStream and they are gorgeous

[Tishka17]

@Zhenay I can suggest trying https://github.com/reagento/dishka/
It has a bit more verbose approach but more features related only to DI. There is an integration with FastStream out of the box if you need it.

[Zhenay]

@Tishka17 Thank you for your suggestion, but we are completely satisfied with FastDepends.
I've heard about Dishka, it's a great and really powerfull tool. But in our case the most important criterion is simplicity and similarity with FastApi DI.