fix: clean up async consumer waiters on cancellation

[nickita-khylkouski]

💡 Motivation and Context

Cancelling _get_or_flush while it is inside asyncio.wait can leave its queue and flush helper tasks pending. Put the wait in try/finally, cancel any unfinished helper tasks, and gather both before propagating cancellation.

💚 How did you test it?

• Full test suite: 2,488 passed, 15 skipped, 38 subtests passed
• Full mypy check: 230 source files, no issues
• Import smoke test and package build
• Added cancellation and result-preservation coverage for _get_or_flush and the worker path

The repository-wide Ruff command currently reports existing formatting and legacy-lint findings in untouched files; the modified Python files pass formatting.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran sampo add to generate a changeset file

An equivalent patch changeset is included at .sampo/changesets/async-consumer-cancellation.md.

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

The contributor directed the scope and exact cleanup behavior. Codex helped inspect the repository, apply the bounded change, and run the repository checks; the implementation cancels only unfinished waiter tasks and gathers both to avoid leaked-task warnings.

[marandaneto]

thanks @nickita-khylkouski