Refactor unexported funcs by removing unnecessary error result

[alexandear]

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• Create test code that corresponds to the modification

The PR removes error result from newMappingNode and createLiteralAndFoldedTokenGroups functions as it is always nil. This increases code coverage and removes unreachable paths.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.46%. Comparing base (7601ad3) to head (2b30585).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #589      +/-   ##
==========================================
+ Coverage   77.37%   77.46%   +0.09%     
==========================================
  Files          21       21              
  Lines        7704     7695       -9     
==========================================
  Hits         5961     5961              
+ Misses       1333     1327       -6     
+ Partials      410      407       -3     

[goccy]

Did you see my comment? I don't require refactoring PR

#555 (comment)