fix: Allocator capsule fallback panic

[orlp]

Another fix for #23965 so if we don't expose an allocator capsule (or under a new name) we don't get a hard error but simply use the fallback.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.61%. Comparing base (18d3e21) to head (22b8a4b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24022      +/-   ##
==========================================
+ Coverage   81.59%   81.61%   +0.01%     
==========================================
  Files        1675     1675              
  Lines      225735   225735              
  Branches     2897     2897              
==========================================
+ Hits       184186   184231      +45     
+ Misses      40821    40776      -45     
  Partials      728      728              

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