fix: annotate context managers with AsyncGenerator for ty 0.0.85

[lesnik512]

Summary

Fixes the scheduled lint-ci failure. Closes #48

ty 0.0.85 bundles a typeshed that deprecates the asynccontextmanager overload for functions annotated -> AsyncIterator[T], so ty check reported two deprecated warnings.

Changes

• modern_di_starlette/main.py: _compose_lifespan's inner composed now returns collections.abc.AsyncGenerator[...].
• tests/test_lifespan.py: same change for user_lifespan; the typing import was left unused and is removed.

collections.abc is used because typing.AsyncGenerator needs both type arguments before Python 3.13.

Checklist

• just lint-ci passes locally with ty 0.0.85 (it failed with the two warnings before the change)
• just test passes locally (15 passed)