Ensure AsyncResolver.close() can be called multiple times

[bdraco]

regressed in #10847 but never in a stable release so I linked the changelog message instead of adding a bugfix.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.77%. Comparing base (b1e9462) to head (3891247).
⚠️ Report is 492 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #10946   +/-   ##
=======================================
  Coverage   98.77%   98.77%           
=======================================
  Files         129      129           
  Lines       39458    39478   +20     
  Branches     2185     2186    +1     
=======================================
+ Hits        38973    38993   +20     
  Misses        337      337           
  Partials      148      148           

Flag	Coverage Δ
CI-GHA	98.65% <100.00%> (+<0.01%)	⬆️
OS-Linux	98.36% <100.00%> (+<0.01%)	⬆️
OS-Windows	96.53% <100.00%> (+<0.01%)	⬆️
OS-macOS	97.50% <100.00%> (+<0.01%)	⬆️
Py-3.10.11	97.40% <100.00%> (-0.01%)	⬇️
Py-3.10.17	97.93% <100.00%> (-0.01%)	⬇️
Py-3.11.12	98.01% <100.00%> (-0.01%)	⬇️
Py-3.11.9	97.48% <100.00%> (+<0.01%)	⬆️
Py-3.12.10	98.41% <100.00%> (-0.01%)	⬇️
Py-3.13.3	98.41% <100.00%> (+<0.01%)	⬆️
Py-3.9.13	97.28% <100.00%> (+<0.01%)	⬆️
Py-3.9.22	97.81% <100.00%> (+<0.01%)	⬆️
Py-pypy7.3.16	79.33% <100.00%> (-18.04%)	⬇️
VM-macos	97.50% <100.00%> (+<0.01%)	⬆️
VM-ubuntu	98.36% <100.00%> (+<0.01%)	⬆️
VM-windows	96.53% <100.00%> (+<0.01%)	⬆️

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[codspeed-hq]

CodSpeed Performance Report

Merging #10946 will not alter performance

_{Comparing async_resolver_close_many (3891247) with master (b1e9462)}

Summary

✅ 59 untouched benchmarks

[patchback]

Backport to 3.12: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.12/3c88f811390cebf524281bebfdbd807a57ab855e/pr-10946

Backported as #10947

🤖 @patchback
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