Codex updated: Fix async/event lifecycle edge cases

[dmulcahey]

This pull request was created with Codex and is an updated version of the pull request it supersedes.

Original pull request: #687

Summary

Fixes async lifecycle edge cases in concurrency limiting, periodic polling cancellation, and async event listener handling.

Failing tests addressed

• tests/test_async_.py::test_gather_with_limited_concurrency_zero_limit_rejected
• tests/test_async_.py::test_periodic_cancellation_propagates
• tests/test_event.py::test_event_base_once_async_multiple_emits_same_tick
• tests/test_event.py::test_event_base_emit_async_callable_object

Verification

• pytest (branch-local targeted run): all listed tests pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.30%. Comparing base (c38e03e) to head (aaf05cd).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #829      +/-   ##
==========================================
+ Coverage   97.27%   97.30%   +0.02%     
==========================================
  Files          55       55              
  Lines       10930    10932       +2     
==========================================
+ Hits        10632    10637       +5     
+ Misses        298      295       -3     

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