chore(headertest): use ErrorIs on original error and NoError for success

[VolodymyrBg]

• Replace assert.ErrorIs(t, errors.Unwrap(err), expected) with assert.ErrorIs(t, err, expected) to rely on errors.Is walking the entire unwrap chain, avoiding brittle single-level unwrapping.
• Use assert.NoError(t, err) for cases expecting no error, which is clearer and idiomatic.
• This makes tests robust against changes in error-wrapping depth and improves readability.

[Wondertan]

@VolodymyrBg, linter complains

[VolodymyrBg]

@Wondertan , corrected

[codecov-commenter]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 35.34%. Comparing base (2469e7a) to head (cd9c9aa). :warning: Report is 583 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4535      +/-   ##
==========================================
- Coverage   44.83%   35.34%   -9.50%     
==========================================
  Files         265      304      +39     
  Lines       14620    24298    +9678     
==========================================
+ Hits         6555     8587    +2032     
- Misses       7313    14759    +7446     
- Partials      752      952     +200     

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