fetch: fix issue 4058

[KhafraDev]

closes #5455
fixes #4058 (tbh didn't even bother reading the ai slop test so can't really confirm 🤷)

[KhafraDev]

The fetch spec is so well written, unironically. This shortcut only works for this once instance of cloning the request, which is why the note was added in #http-network-or-cache-fetch and not #concept-request-clone.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.46%. Comparing base (0f1f890) to head (f9d6e57).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5456   +/-   ##
=======================================
  Coverage   93.46%   93.46%           
=======================================
  Files         110      110           
  Lines       37106    37115    +9     
=======================================
+ Hits        34682    34691    +9     
  Misses       2424     2424           

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[mcollina]

lgtm