fix: annotate context managers with Generator for ty 0.0.85

[lesnik512]

Summary

Closes #277

ty 0.0.85 bundles a typeshed that deprecates the contextmanager and asynccontextmanager overloads for functions annotated -> Iterator[T] or -> AsyncIterator[T]. ty check reported 9 such warnings, which failed the scheduled lint-ci run. This annotates those functions with Generator[T] and AsyncGenerator[T].

They come from collections.abc, because typing.Generator needs all three type arguments before Python 3.13.

Changes

• lite_bootstrap/bootstrappers/fastapi_bootstrapper.py, fastmcp_bootstrapper.py: lifespan context managers return AsyncGenerator[...]
• lite_bootstrap/exceptions.py: capture and collect_teardown_errors return Generator[...]; drop the now unused typing import
• tests/conftest.py, tests/test_fastapi_bootstrap.py, tests/test_litestar_bootstrap.py: test context managers return Generator[...]

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