chore: add refreshTokenSet to simplify getTokenSet()

[frederikprijck]

Moving logic related to doing the refresh_token grant in a separate method to improve readability.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.96%. Comparing base (05ec221) to head (5a6a8e8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2403      +/-   ##
==========================================
+ Coverage   87.90%   87.96%   +0.05%     
==========================================
  Files          37       37              
  Lines        4127     4147      +20     
  Branches      818      822       +4     
==========================================
+ Hits         3628     3648      +20     
  Misses        496      496              
  Partials        3        3              

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