Fix once-only and awaitable event listeners

[dmulcahey]

An asynchronous once() listener is not unsubscribed until its task runs, so two immediate emits can invoke it twice. Reentrant emits can also revisit a listener captured by an outer emit. Callable objects with asynchronous __call__, and synchronous callbacks returning awaitables, are currently left unscheduled.

Consume once-listeners synchronously and schedule/track the value returned by a callback when it is awaitable. ensure_future() accepts both coroutines and existing futures/tasks. Regression tests cover back-to-back and reentrant emits, async callable objects, and returned futures/tasks.

Supersedes #687. This refresh is limited to event delivery; it drops the unrelated concurrency-limit validation and periodic-cancellation behavior changes.

Validation against dev at 66603431339afe37fa0048b70ff31d77dceb8f95: Python 3.12 full suite, 1387 passed, coverage above the 95% project gate; full pre-commit (codespell, Ruff, formatting, mypy, lock check) passed. Relevant regression checks fail on the unchanged base. GitHub CI for Python 3.12/3.13/3.14 is reported separately on the PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.19%. Comparing base (6660343) to head (4691fd0).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #829   +/-   ##
=======================================
  Coverage   97.19%   97.19%           
=======================================
  Files          57       57           
  Lines       10560    10560           
=======================================
  Hits        10264    10264           
  Misses        296      296           

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