implement flake8_async ASYNC102 await in finally or cancelled

[altendky]

Summary

When working with async cleanup activities it is important to be aware of shielding the cleanup from cancellation.

https://anyio.readthedocs.io/en/stable/cancellation.html#shielding

This rule looks for unshielded awaits in relevant cleanup paths such as exception handling and finally blocks. It walks the AST looking for awaits but prunes when it finds shielding context managers.

Test Plan

Draft For:

• review TODOs
• consider if this is the ruff implementation of https://flake8-async.readthedocs.io/en/latest/rules.html#async102 (at a glance the answer seems to be yes)
• explicitly test async for and async with
• review # noqa: ASYNC102 - fixthis which are mostly around direct assignment to the .shield attribute
• revisit linter message, it's just shield it! right now
• don't trigger on anyio and trio.aclose_forcefully()
• review all panics and either implement or handle by having no output
• handle .__aexit__()
• require a timeout deadline

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.