Refactor header count logic for accurate line tracking

[wneessen]

Changed headerCount from uint to int and updated its logic to accurately count lines written in headers. Modified writeHeader to return the number of lines written, ensuring better alignment between header tracking and actual output. This fixes a bug with S/MIME signing if a header line spanned over more than one line.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.70%. Comparing base (559046e) to head (077c438).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #421   +/-   ##
=======================================
  Coverage   94.70%   94.70%           
=======================================
  Files          30       30           
  Lines        3532     3534    +2     
=======================================
+ Hits         3345     3347    +2     
  Misses        138      138           
  Partials       49       49           

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