fix: annotate context managers with Generator for ty 0.0.85

[lesnik512]

Summary

The scheduled CI run fails because ty 0.0.85 reports a deprecated warning on _rpc_child in tests/test_inject.py. Its bundled typeshed deprecates the contextmanager overload for functions annotated -> Iterator[T].

Closes #40

Changes

• Annotate _rpc_child as -> collections.abc.Generator[Container] instead of typing.Iterator[Container]. collections.abc is used because typing.Generator needs all three type arguments before Python 3.13.

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