fix: annotate context managers with AsyncGenerator for ty 0.0.85

[lesnik512]

Summary

Fixes the scheduled lint-ci failure. Closes #67

ty 0.0.85 ships a typeshed that deprecates the asynccontextmanager overload for functions annotated -> AsyncIterator[T], so ty check now warns on both lifespan functions. Annotating them -> AsyncGenerator[T] uses the non-deprecated overload. The type comes from collections.abc because typing.AsyncGenerator needs both type arguments before Python 3.13.

Changes

• modern_di_fastapi/main.py: the composed lifespan returns collections.abc.AsyncGenerator[...] instead of typing.AsyncIterator[...].
• tests/test_lifespan.py: same change for user_lifespan.

Checklist

• Lint and format pass (ruff)
• Type check passes (ty)
• Tests pass and new behavior is covered
• Build succeeds (uv build) if packaging or build config changed
• Repo metadata stays consistent across the three surfaces (GitHub description, pyproject description, profile blurb) if this touches packaging