Move end-to-end test HTTP server to a thread to support 3.14+

[aiudirog]

This is an alternative to #61 & #63 to instead move the testing HTTP server to a thread in the same process.

In addition to avoiding to multiprocessing migration from fork to spawn entirely, it also provides a few other benefits such as not needing to rebind the server port and proper shutdown of the server instead of just terminating the child process.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.48%. Comparing base (912c9ec) to head (f560ee7).
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Additional details and impacted files

@@           Coverage Diff           @@
##             main      #65   +/-   ##
=======================================
  Coverage   95.48%   95.48%           
=======================================
  Files           3        3           
  Lines         155      155           
=======================================
  Hits          148      148           
  Misses          7        7           

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