Add code coverage for DataGridViewComboBoxColumn

[Zheng-Li01]

Related #13442

Proposed changes

Add unit test file:DataGridViewComboBoxColumnTest.cs for public methods of the DataGridViewComboBoxColumn.cs

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[codecov]

Codecov Report

Attention: Patch coverage is 99.77578% with 1 line in your changes missing coverage. Please review.

Project coverage is 76.65620%. Comparing base (ece6f6d) to head (ad80cd1).

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #13617         +/-   ##
===================================================
+ Coverage   76.58723%   76.65620%   +0.06896%     
===================================================
  Files           3253        3254          +1     
  Lines         640992      641438        +446     
  Branches       47439       47460         +21     
===================================================
+ Hits          490918      491702        +784     
+ Misses        146541      146204        -337     
+ Partials        3533        3532          -1     

Flag	Coverage Δ
Debug	76.65620% <99.77578%> (+0.06896%)	⬆️
integration	18.48216% <ø> (ø)
production	51.13956% <ø> (+0.11781%)	⬆️
test	97.41473% <99.77578%> (+0.00298%)	⬆️
unit	48.55183% <ø> (+0.11990%)	⬆️

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