[PR #8611/1fcef940 backport][3.10] Fix handler waiting on shutdown

[patchback]

This is a backport of PR #8611 as merged into master (1fcef94).

There seems to be some kind of race condition in certain circumstance which can lead to .wait_for_disconnection() being called after the request has already been cancelled.

Additionally, I see no guarantee that when .wait_for_disconnection() completes, that the handler itself has completed (and not suppressed a cancellation etc.).

However, I couldn't figure out a shutdown test in test_run_app.py that triggers this edge case.

[codecov]

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@@           Coverage Diff           @@
##             3.10    #8627   +/-   ##
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  Coverage        ?   97.57%           
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  Files           ?      108           
  Lines           ?    33678           
  Branches        ?     4040           
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  Hits            ?    32861           
  Misses          ?      597           
  Partials        ?      220           

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