Add cooperative signal timeouts

[tamird]

A signal timeout can interrupt asyncio while it schedules a task's next step, leaving a pending task with no continuation 1. Catching the resulting failure and cancelling that task cannot reliably recover it.

Add opt-in cooperative signal timeouts through the proposed pytest-timeout expiry hook 2. The signal handler records the supplied failure and queues cancellation without raising into the scheduler. The owned runner then reports the timeout with the interrupted await's traceback, while preserving process-control exceptions and external cancellation. Final runner shutdown has a separate interruption path that closes the loop and may leave resource cleanup incomplete.

Existing signal behavior remains the default. Cooperative cancellation cannot stop an indefinitely blocking callback or a task that refuses to finish; pytest-timeout's thread method remains the hard process-stop option. This consumer does not take over other async plugins, synchronous tests that call asyncio.run(), retry budgets, or background-task ownership.

This keeps runner integration separate from timer policy, as requested in the earlier timeout discussion 3. The dedicated integration environment pins the producer prerequisite; that pin should become a released test dependency once the hook is available. Related to #215.

Prepared with Codex assistance.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.00000% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.38%. Comparing base (70815a7) to head (58aed64).

Files with missing lines	Patch %	Lines
pytest_asyncio/_timeout.py	92.98%	3 Missing and 5 partials ⚠️
pytest_asyncio/plugin.py	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1548      +/-   ##
==========================================
+ Coverage   94.50%   95.38%   +0.87%     
==========================================
  Files           2        3       +1     
  Lines         510      628     +118     
  Branches       62       78      +16     
==========================================
+ Hits          482      599     +117     
+ Misses         22       19       -3     
- Partials        6       10       +4     

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