fix: annotate context managers with Generator for ty 0.0.85

[lesnik512]

Summary

Closes #68

ty 0.0.85 ships a typeshed that deprecates the asynccontextmanager overload for functions annotated -> AsyncIterator[T], so ty check in the scheduled run now reports a warning and fails. This annotates the lifespan manager as AsyncGenerator[None] instead.

AsyncGenerator comes from collections.abc because typing.Generator needs all three type arguments before Python 3.13.

Changes

• modern_di_litestar/main.py: _lifespan_manager returns collections.abc.AsyncGenerator[None] instead of typing.AsyncIterator[None].

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