fix: remove fallback 500 status for client errors

[kricsleo]

resolves #495

This PR removes the fallback 500 status for client errors.
❗ Noted that this might be a breaking change for users who rely on this fallback behavior.

When 500 is included in the retryStatusCodes (default):
Before and after are the same.

When 500 is excluded (manually):
Before: won't retry
After: will retry

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 69.82%. Comparing base (27996d3) to head (03a02e2).
Report is 67 commits behind head on main.

Files with missing lines	Patch %	Lines
src/fetch.ts	75.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #496       +/-   ##
===========================================
+ Coverage   56.86%   69.82%   +12.96%     
===========================================
  Files          16       17        +1     
  Lines         728      527      -201     
  Branches      113      137       +24     
===========================================
- Hits          414      368       -46     
+ Misses        303      148      -155     
  Partials       11       11               

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